The activity of metronidazole was studied by the diffusion method using two discs on a model group of 40 anaerobic bacterial strains isolated recently from clinical specimens. The minimal inhibitory concentrations (MICs) were determined by the dilution method and regression curves by the method of minimum square errors. Some 82.5% of bacteria of the model group were susceptible to metronidazole. All the routinely isolated anaerobes (582 strains) were tested with metronidazole over a period of 20 months. Susceptibility was recorded in 469 strains (80.6%). The clinical study was carried out on two groups of inpatients with bacteriologically confirmed mixed aerobic and anaerobic infections. Subjects in Group 1 (25 patients) were given clindamycin and gentamicin and those in Group 2 (30 patients) metronidazole and gentamicin. The results showed that the two combinations of drugs had comparable effects.