Cefazolin was used in the treatment of nine patients with acute enteric fever proven by positive blood cultures. In seven patients the causative organism was Salmonella typhi and in two it was Salmonella paratyphi B. Minimal inhibitory and minimal bactericidal concentrations of cefazolin against the nine isolates ranged between 1.95 and 3.90 mug/ml. Cefazolin was administered either intramuscularly or intravenously in a daily dose of 3 to 6 g for 11 to 16 days. The mean peak serum antibiotic concentration after a 0.5-g intravenous injection was 64.4 mug/ml, and the mean trough concentration, 3 h later, was 12.7 mug/ml. The highest serum inhibitory dilution at peak level was frequently 1/64, and at trough level it was 1/16 to 1/32. The acute infection was satisfactorily controlled in all patients. Phlebitis, complicating intravenous therapy, in five out of eight patients, was the only side effect observed. Relapse of typhoid fever, as documented by positive blood culture, occurred in one patient 11 days after treatment course was completed. More extensive clinical studies are required before drawing any conclusions regarding the efficacy of cefazolin in acute enteric fever.