Cefoxitin, a new cephamycin antibiotic that is active against aerobic and anaerobic bacteria, was studied by 35 investigators in the United States. Of 657 patients eligible for evaluation of efficacy of the compound, 69% were cured and 92% were cured or improved on clinical grounds. Bacteriologic response to therapy with cefoxitin was equally good for infections due to gram-positive cocci (94% cured), gram-negative bacilli (87% cured), and anaerobes (95% cured). Cefoxitin was effective clinically and bacteriologically in the eradication of infections due to organisms resistant to ampicillin, cephalothin, chloramphenicol, tetracycline, and aminoglycosides. Overall rates of favorable response to cefoxitin therapy by disease were: lower respiratory tract infections, 90%; urinary tract infections, 87%; intraabdominal infections, 90%; gynecologic infections, 94%; and septicemia, 84%. Cefoxitin was tolerated well, and major abnormalities of hematologic, hepatic, renal, or central nervous system function were encountered rarely. Resistance to cefoxitin did not develop among gram-negative cocci, anaerobes, or gram-negative bacilli in the medical centers in which the antibiotic was used.