An open, randomized, culture-controlled clinical study was designed to compare the efficacy of a single 2 g dose of metronidazole (Elyzol) with standard 7-day therapy in the treatment of bacterial vaginosis (BV). Forty-one of 47 (87%) patients given the single dose and 30 of 33 (91%) given the 7-day treatment were found to be cured seven days after treatment. At final assessment, 24 of 34 (71%) patients given the single dose and 22 of 28 (79%) given the 7-day treatment remained cured. The two regimes were equally efficaceous in eradicating Gardnerella vaginalis, Bacteroides spp. and Mobiluncus spp. (anaerobic curved rods) from vaginal specimens from patients with BV. The in-vitro activity of metronidazole and its hydroxy metabolite was determined for 11 strains of Gard. vaginalis, 17 strains of Bacteroides spp. and 14 strains of Mobiluncus spp. which had been isolated from patients prior to treatment. The MIC of metronidazole against Gard. vaginalis varied between 2 and greater than or equal to 128 mg/l (median MIC 32 mg/l), but the hydroxy metabolite showed a markedly increased activity against eight of the strains tested (median MIC 4 mg/l). The MIC of metronidazole against the Mobiluncus spp. varied between 0.5 and greater than or equal to 128 mg/l (median MIC 16 mg/l) and the hydroxy metabolite showed little increased activity (median MIC also 16 mg/l). The Bacteroides organisms were highly susceptible to metronidazole and to the hydroxy metabolite, each having a median MIC of 1 mg/l.