We believe we have shown in the animal model that in cancer the host's lymphoreticular tissues are engaged by a systemically disseminated antilymphocytic humoral factor that destroys the T-cell-dependent pathway, allowing the dissemination of the disease that results in the death of the animal. From studies on patients with cancer there appears to be a real possibility of the presence of a similar substance in man. Such a factor may form the basis for the definiation of cancer and an explanation of cancer illness on the runt hypothesis. Moreover, if the factor could be identified and antagonized a major therapeutic advantage would be achieved.