The rate of bactericidal activity and inactivation of metronidazole was studied in time-kill curves with Haemophilus vaginalis (Corynebacterium vaginale). The minimum inhibitory concentrations of metronidazole for the eight strains tested ranged from 4 to 16 micrograms/ml. At a concentration of 20 micrograms/ml, metronidazole demonstrated a slow cidal effect against exponential-phase organisms, requiring 24 to 48 h for completion. Inactivation of metronidazole during the time-kill curve was quite variable and averaged 28% of the starting concentration after 48 h. Against stationary-phase organisms (inoculum, 10(10) to 10(11) colony-forming units per ml), a slow cidal effect was also seen, with an average inactivation of metronidazole of 38% after 48 h. At a subinhibitory concentration of 5 micrograms/ml, metronidazole was inactivated to the greatest degree (57% after 48 h). Therefore, in contrast to earlier studies in which metronidazole was rapidly and consistently cidal within 4 h against obligate anaerobes and was almost completely inactivated by 8 h, the bactericidal effect of metronidazole againsts H. vaginalis in this study was much slower and was associated with a variable and slower rate of inactivation.