The results of the experimental analysis reported in this and the two preceding papers (10, 11) indicate that in murine pneumococcal infections penicillin per se destroys the invading organisms only in those parts of the lesions where the bacteria are multiplying rapidly and are thus maximally susceptible to the bactericidal action of the drug. In areas where the bacterial growth rate is slowed, either because the pneumococci have reached a maximum population density, or because the accumulated exudate affords a relatively poor medium for rapid growth, the destructive effect of the antibiotic is greatly diminished. In such portions of the lessions the cellular defenses of the host are observed to play a major role in eliminating the bacteria. In sites where frank suppuration has developed, however, even the combined actions of the penicillin and the cellular defenses of the host are relatively ineffective in ridding the tissues of bacteria. Here, because of the poor medium provided by the pus, the pneumococci remain metabolically sluggish and therefore are not killed rapidly by the penicillin. At the same time the leucocytes in the necrotic exudate have deteriorated to the point where they cannot effectively perform their phagocytic functions. As a result, bacteria persist in such lesions for many days in spite of the most intensive penicillin treatment administered both locally and systemically. A strict analogy cannot be drawn between the action of penicillin upon specific pneumococcal lesions produced in the laboratory and its effect upon acute bacterial infections in man. Host-parasite relationships in acute bacterial infections are determined not only by the strain of parasite and the specific host involved, but also by the site in the body at which the infection occurs (16). Nevertheless, in spite of the number of variables involved, it may be possible, by means of selected laboratory models, to illustrate general principles of infection which in all probability apply to human disease. Bearing in mind the limitations of the methods employed in the present experiments, it would appear justifiable to draw the following conclusions concerning the clinical use of penicillin in acute infections caused by penicillin-sensitive bacteria. The earlier that treatment is begun the more likely is penicillin to effectuate a rapid cure. When therapy is started before the bacteria have reached a maximum population density in any part of the lesion, and before a cellular exudate is formed, the great majority of the infecting organisms will be in a state of active multiplication and thus will be killed promptiy by the bactericidal action of the drug. If, on the other hand, treatment is delayed until the bacterial growth has attained its maximum in older parts of the lesion, and the inflammatory reaction has become well advanced, the resultant slowing of bacterial metabolism will so interfere with the bactericidal action of the penicillin that ultimate destruction of many of the bacteria will have to depend upon the slower clearing effect of the phagocytic cells. In such instances of delayed therapy specific antibody, which is formed relatively slowly, may play an important role in recovery (6). If relapse is to be avoided, however, penicillin therapy must often be continued longer in well established infections than in those treated at a very early stage. Still further delay in treating infections which are prone to cause tissue destruction and suppuration, may lead to the establishment of abscesses. Fully developed abscesses often will not respond to chemotherapy alone; they will ultimately require drainage. As shown by the present murine experiments, the relative ineffectiveness of penicillin under these circumstances is due not only to the failure of the drug to kill the metabolically sluggish bacteria surviving in the pus, but also to the ineffectiveness of the phagocytic cells, most of which are non-motile or dead. Even if specific antibody gains access to such purulent foci, many of the bacteria will continue to survive because of the degenerated state of the leucocytes. It is evident, therefore, that the stage of the infection at which penicillin treatment is begun is often crucial. Equally critical may be the location of the infection. Bacterial lesions in different sites of the body vary greatly in their responses to penicillin therapy. This inconstancy of therapeutic effectiveness is due primarily to the participation of host factors of defense which differ widely in various tissues and at the same time play a major role in the curative action of the antibiotic. In cases of pneumococcal pneumonia, for example, in which each milliliter of the patient's blood contains more than 1000 pneumococci, blood cultures may become negative in a matter of minutes after the start of intensive treatment (17). The remarkable promptness with which penicillin therapy controls such acute bacteriemia is due, first, to its suppressive effect upon the primary infection in the lungs and regional lymph nodes from which the bacteria are being poured into the blood stream (16) and, secondly, to its synergistic action with the cellular defenses of the circulation. The latter are known to be extraordinarily efficient, perhaps more so than in any other tissue of the body (18). Assisting them in destroying the circulating bacteria is the penicillin's own bactericidal effect, which operates rapidly upon the metabolically active organisms in the plasma. Rarely, if ever, as they often do in other tissues of the body (10), do bacteria in the bloodstream reach such numbers, or do inflammatory cells accumulate intravascularly to such an extent, as to create metabolic conditions which depress the bactericidal actions of the antibiotic. In contrast, more prolonged and extensive penicillin therapy is needed to cure pneumococcal endocarditis (19), meningitis (19, 20), or infections of the serous cavities (3, 4). The cellular defenses of the heart valves and of the "open" fluid-containing cavities of the body are relatively inefficient as compared to those that operate in the bloodstream and in tissues with tightiy knit architectures such as the lungs and lymph nodes (16). In endocarditis relatively few phagocytic cells ever reach the site of the offending bacteria (21), and in infections of fluid-containing cavities, the phagocytic efficiency of the mobilized leucocytes is seriously interfered with by the "dilution effect" of the fluid (22, 23). Accordingly, final destruction of the bacteria must depend primarily upon the bactericidal effect of the antibiotic itself, since little assistance is provided by phagocytosis. It is no wonder, therefore, that such infections, as compared to bacteriemia, are relatively refractory to penicillin therapy. Certainly penicillin, in spite of its remarkable therapeutic properties, falls far short of being a therapia sterilans magna (24). Its effectiveness does not depend solely upon the inherent susceptibility of the infecting agent to its antimicrobial action. How readily it will cure a given infection is determined also by the state of growth of the bacteria in the various zones of the lesions, the influence of the purulent exudate upon the bactericidal action of the drug, and the destructive effect of the inflammatory phagocytes upon the invading bacteria. Optimal use of penicillin as a therapeutic agent requires due consideration of all of these factors. Finally, it should be emphasized that the conclusions drawn from this experimental analysis cannot be applied to antibiotic therapy in general. They pertain only to the action of penicillin in acute infections caused by penicillin-sensitive bacteria which act in the host as extracellular parasites (16). The most common human infections included in this category are those caused by pneumococci and Group A beta hemolytic streptococci.7 Whether they apply also to infections due to penicillin-sensitive staphylococci may be questioned because of recent evidence that certain pathogenic strains will survive phagocytosis (27). In diseases such as tuberculosis, brucellosis, and typhoid fever, which are treated with antibiotics having properties different from those of penicillin (28) and which are caused by bacteria capable of intracellular parasitism (28), factors other than those considered in the present analysis must certainly be involved in the curative effect of antimicrobial therapy.