Recent developments that influence patterns of antibiotic prescription for obstetric-gynecologic patients include a better understanding of the multibacterial dimensions of pelvic infections, the introduction of new antibiotics, and the pressures for cost-containment in medical care. Prophylaxis has become established as effective for prevention of infection following vaginal hysterectomy and cesarean section, but its success in abdominal hysterectomy has been less uniform. For patients with pelvic infections, the poorest clinical response occurs in those whose infection is well established before initiation of therapy. Because gram-negative anaerobic bacteria often are present in such infections, the selection of antibiotics should be based on the susceptibility patterns of the infecting organism(s) and on the ability of the agent to reduce high counts of anaerobic bacteria. Both metronidazole and clindamycin meet these criteria. Controlled studies of infections seen early in the clinical course are few. The initial selection of agents effective against gram-negative anaerobes seems important in the treatment of endomyometritis following cesarean section, whereas curettage seems the most significant therapy for infections following abortion.