The various effects have been examined against the anaerobes by administering tinidazole when the operations of gynecology and obstetrics and the following results were obtained. 1. The serum concentrations after the single administration of 1,000 mg of TDZ showed 56.6 micrograms/ml of the peak value (3 hours) and 27.7 micrograms/ml of 24 hours value, and the antibacterial potency was maintained for a long duration. The concentration in the vaginal secretions was 21.8 micrograms/tampon within 24 hours. 2. The detections of anaerobes were made in 74 cases, before the administration, with the results of 1 case alone and 73 cases mixed with aerobes, 10 species and 133 strains being detected, which comprise 33 cases with 1 species, 27 cases with 2 species mixed, 10 cases with 3 species mixed and 4 cases with 4 species mixed. The main species detected were Bacteroides fragilis in 46 strains (34.6%), Peptostreptococcus spp. in 31 strains (23.3%) and Peptococcus spp. in 26 strains (19.5%). 3. The MIC values of the detected bacteria to the TDZ centered upon 0.78 micrograms/ml for Bacteroides fragilis, 3.13 micrograms/ml for Peptococcus spp. and 0.78 micrograms/ml for Peptostreptococcus spp. and those of the others were low, a lot of those being of sensitive strains. 4. The bacteriological effects on the anaerobes were, after the medication, observed to be 71.4% and found significantly more effective than 33.3% of natural eradication and decrease observed in the not administered group. 5. Clinically, no anaerobic infections were observed often operation and the therapeutical effects were observed. A little side effect was observed. 6. According to the above-mentioned results, it was observed possible to prevent from the anaerobic infections after operation in gynecology and obstetrics by administration of TDZ.