Comparative bactericidal activities were determined utilizing a relatively large number of test strains, in both agar and broth media, with special reference to the time of exposure of the bacteria to certain beta-lactam antibiotics. It was apparent that the activities increase with time. The concentrations producing a 99.9% kill with cephalothin for Escherichia coli, Klebsiella sp., and carbenicillin for Pseudomonas aeruginosa were higher in broth than in agar. In contrast, those of benzylpenicillin for alpha-streptococcus (non-enterococcal) were higher in agar than in broth. If the bactericidal concentrations with 3-hour or 6-hour exposure to antibiotics were used as the criterion, these concentrations of carbenicillin for P. aeruginosa, and benzylpenicillin for alpha-streptococcus were, in particular, unusually high compared with the conventionally determined bacteriostatic concentrations (MICs).