Current anaerobic susceptibility tests have interpretive problems. We placed a Bioscreen C analyzer in an anaerobic chamber for broth microdilution testing. The Bioscreen C can automatically dispense broth, antibiotics and inoculum, then incubate and spectrophotometrically monitor growth. Antimicrobial agents (cefoxitin, ceftizoxime, clindamycin, ciprofloxacin, and imipenem) were diluted in brucella broth (BB). Organisms (Bacteroides fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741) were grown on brucella blood agar plages and dispensed to give 1 000 000 CFU/ml. Tests were done in triplicate in BB with turbidity measurements made every 30 min for 48 h using a wide band filter (420-580 nm). Growth curves for B. fragilis and B. thetaiotaomicron in control wells were characterized by 5 h lag and logarithmic phases, with the start of stationary phase at 10-11 h. Increasing concentrations of antimicrobial agents resulted in increased lag phases. The lag phase for B. Fragilis was 20 h and 28 h at clindamycin concentrations of 0.25 and 0.5 ug/ml, respectively. For B. thetaiotaomicron and imipenem, small initial increases in turbidity were followed by secondary lag phases of 20, 27 and 38 h at concentrations of 0.06, 0.12 and 0.25 ug/ml, respectively. The transition from lag to log phases for these organisms may have been caused by decreasing agent concentrations and/or resistant subpopulations. Clearly, microdilution MIC values for these agents depend on when the test was read and not on an intrinsic or tru MIC value.