A new cephalosporin antibiotic, cefazolin, was administered by im injection to fifty patients with urinary tract infection. Dosage was 2–4 g per day for an average of eight days. The infecting organism was eradicated from 47 of the 50 patients, although superinfection with a resistant organism occurred in five instances. Relapse of infection occurred in five of the 42 patients with a negative urine culture after treatment. Therefore, the final rate of cure was 74%. Side effects and toxicity were minimal.