Clindamycin, rosamicin, josamycin, and metronidazole had similar inhibitory activity against 29 clinical isolates of Bacteroides fragilis, i.e., 100% of strains were inhibited by 0.8 microng of metronidazole or josamycin/ml and 100% by 1.6 microng of clindamycin or rosamicin/ml. Metronidazole was bactericidal against 97% of the isolates, and clindamycin or rosamicin (in concentrations of 1.6 microng/ml) was bactericidal against 80%. Erythromycin and josamicin were the least bactericidal agents in vitro. Thirty-two patients with pleuropulmonary and intraabdominal or pelvic infections caused by anaerobic bacteria were treated with clindamycin. Cure was achieved in 27 patients. In another group of 37 patients treated with parenteral clindamycin, diarrhea developed in 30% and was significantly more common in those patients with abdominal or pelvic infection. Only one patient developed pseudomembranous colitis. These observations suggest that clindamycin is an excellent and relatively safe antibiotic for treatment of infections caused by anaerobes when combined with surgery or with other antibiotics selected for activity against aerobic gram-negative bacilli.