A single drug, clindamycin phosphate, was administered to 22 seriously ill patients with anaerobic infections. Ten patients had only anaerobes with no facultative bacteria in closed space infections. All had a satisfactory response to therapy, but one developed a Pseudomonas suprainfection two weeks later. Twelve patients had intra-abdominal or pelvic abscesses that contained mixed aerobes and anaerobes. Each patient had an average of two aerobes (range one to four) and three anaerobes (range one to six). All 12 patients had failed to respond to previous antibiotics. After starting clindamycin therapy alone all patients displayed a satisfactory response. Anaerobes could not be cultured after treatment, although aerobes usually persisted. Our experience indicates that clindamycin is an excellent drug for anaerobic infections. Furthermore, its successful use in mixed aerobic-anaerobic abdominal infections suggests that anaerobes play a decisive role in this situation.