Cefsulodin (SCE-129) is a cephalosporin with a spectrum of antibacterial activity largely limited to Pseudomonas aeruginosa and Staphylococcus aureus. Cefsulodin was compared with carbenicillin, ticarcillin, mezlocillin, and piperacillin against 779 nonenteric gram-negative bacilli and staphylococci collected from five geographically separate institutions. Against P. aeruginosa, cefsulodin was somewhat more active than piperacillin and much more active than other penicillins. In addition, cefsulodin was active against penicillinase-producing strains of S. aureus. Collaborative efforts in three laboratories led to the following tentative zone size breakpoints for 30-micrograms cefsulodin disks: susceptible greater than or equal to 18 mm (minimal inhibitory concentration) [MIC] less than or equal to 16 micrograms/ml) and resistant less than or equal to 14 mm (MIC greater than or equal to 64 micrograms/ml). These zone standards are still tentative since the dosage schedule has not yet been defined and sufficient clinical experience has not yet been gathered to support the validity of these MIC breakpoints.