OBJECTIVE: To study the postantibiotic effect (PAE) for Bacteroides fragilis after exposure to common anaerobic antimicrobials with two different methods, by viable counting and by measuring CO2 generation in a BACTEC(R) blood culture system. METHODS: Four strains of B. fragilis were exposed for 1, 2 and 4 h to cefoxitin, chloramphenicol, clindamycin, imipenem or metronidazole at concentrations from 1 to 16 x MIC. The drugs were removed by dilution into BACTEC 7A(R) vials and growth determined with viability counts and CO2 production. RESULTS: The durations of the PAEs determined by the two methods correlated well (r=0.913, p<0.005). PAEs of up to 4-5 h were induced by imipenem and metronidazole with achievable concentrations and exposure durations. Chloramphenicol induced short or no PAEs, but cefoxitin and clindamycin induced PAEs up to 2 h with high AUC values. The imipenem PAEs and the short cefoxitin and clindamycin PAEs were dependent on AUC. CONCLUSIONS: Significant PAEs against B. fragilis were induced by imipenem and metronidazole. Determining PAE by measuring CO2 production is an accurate and less time-consuming alternative to the conventional method of viable counts.