The new antiprotozoal agent, tinidazole, was found to be bactericidal against all 52 isolates of obligate anaerobic bacteria tested, 42 Bacteroides fragilis, 4 clostridia and 6 peptostreptococci. The minimum bactericidal concentrations of tinidazole for B. fragilis ranged from 0.25 to 4 μglml, and those of metronidazole from 0.25 to 8 μg/ml, i.e. several times lower than the serum concentrations achievable after oral administration. In most cases the MIC was identical with MBC or half of it. On the average, tinidazole was slightly more effective against B. fragilis than metronidazole. Although essentially the activities of the two drugs were positively correlated, there was a fourfold difference in their MBC for 10 of the 42 B. fragilis.