A new antibiotic, cephalothin, was shown, in vitro, to have a broad spectrum of activity, including penicillin-resistant staphylococci.Pseudomonasstrains were totally resistant. Therapeutic concentrations were achieved with doses of 0.5 to 1 gm given intramuscularly every four to six hours or 4 to 6 gm daily in divided dose or by sustained infusion intravenously. Sixty per cent to 90% of the dose appeared in the urine; however, there was no evidence of accumulation or toxicity in patients with poor renal function, possibly because of rapid tissue destruction. Good or excellent clinical results were seen in 51 of 61 infections caused by gram-positive or gram-negative organisms. Patients with a history of penicillin allergy were given cephalothin with no resultant difficulty.