The effectiveness of cefoxitin sodium, a new semi-synthetic cephamycin, was evaluated from the results of administration of the drug i.v. to 72 patients hospitalized for a variety of severe infections. With a dose range of 2 to 12 g/day, pathogens in various infections were eradicated within 48 h. A therapeutic response was obtained against some cephalothin-resistant pathogens. Fifty additional patients with more moderate infections were treated with cefoxitin i.m., 3 to 6 g/day. Satisfactory responses were obtained in most cases within 48 h, although recurrent Escherichia coli infections of the urinary tract occurred.