Parenteral therapy with gentamicin, cloxacillin, ampicillin and cephalothin was surveyed on a surgical, a gynecologic and medical ward of a teaching hospital. During a 3-month period 219 patients (12.9% of the total number admitted to the three wards) received at least one of the four antibiotics parenterally. Ampicillin and gentamicin were used most frequently on the three wards when the indication for therapy was either infection or empirical use. Cephalothin was used most frequently for prophylaxis in the gynecologic and surgical patients; no medical patient received this drug. Overall, therapy was assessed to be irrational in 42.0, 50.0 and 12.0% of the surgical, gynecologic and medical patients, respectively. Prophylaxis was the indication for therapy in 76.9 and 86.8% of the surgical and gynecologic patients, respectively, for whom the therapy was assessed to be irrational.