The comparative bactericidal activity of penicillin G, carbenicillin, clindamycin, and metronidazole against eight susceptible strains of Bacteroides fragilis and four strains of Clostridium perfringens was determined by performing colony counts anaerobically of cultures incubated in brucella broth. With the B. fragilis strains, there was a lag phase of growth of approximately 8 h, during which time metronidazole did not reduce the colony counts. However, within 4 h of the onset of exponential growth, metronidazole caused an abrupt decrease in counts to less than 100 colonies per ml in all strains tested. Moreover, in two strains in which the bactericidal rate was followed hourly, a 3- to 6-log decrease occurred over 1 h or less. In contrast, penicillin G and carbenicillin caused a gradual decline in colony counts from the start of approximately 1 log for each 8-h interval and were bactericidal for all strains tested. Clindamycin demonstrated the slowest bactericidal activity and for 25% of the strains was only bacteriostatic. With the C. perfringens strains, after a lag phase of 4 h, an abrupt decrease in colony counts also occurred with metronidazole, whereas penicillin and clindamycin again demonstrated more gradual killing effects. These studies showed a unique, time-related bactericidal action of metronidazole as compared with the other three antimicrobial agents.