The influence of metronidazole on some strains of facultatively anaerobic bacteria was studied. In high doses similar to those used for local treatment the growth of some strains was clearly suppressed, moreover there was an inhibition of hemolysis in hemolytic strains even when they were found to be resistant to the drug. Results are of particular importance for clinical problems as they might explain a number of alterations in the microbial flora of the vagina during and after therapy with metronidazole.