Thirty-one adult patients with infections due to anaerobic bacteria were treated with cefamandole. Bacteroides fragilis group (17) and Bacteroides melaninogenicus (13) were the most frequent anaerobes isolated. Duration of therapy varied from 2 to 49 days. Results were judged satisfactory in 26 cases, and unsatisfactory in 1 case. Four cases could not be evaluated. Adverse reactions occurred in 16 patients and included positive direct Coomb's test without hemolysis, transient liver function abnormalities, phlebitis, reversible neutropenia, fever, eosinophilia, and toxic epidermal necrolysis. The more significant reactions were associated with prolonged therapy. None was lethal. These data suggest that cefamandole is effective in treatment of most anaerobic infections.