The use of antibiotics in the management of preterm, premature rupture of membranes remains controversial. By use of a prospective randomized double-blind design we investigated the maternal-fetal benefits associated with antibiotic therapy in 85 women with premature rupture of membranes at 34 weeks' estimated gestational age. In the treatment group 40 patients received intravenous mezlocillin for 48 hours followed by oral ampicillin until delivery. In the control group 45 patients received intravenous and oral placebo. Patients who received antibiotics had chorioamnionitis and endometritis less frequently than the control group (p less than 0.01 and p less than 0.05). Pathologic examination of the placentas showed a lower incidence of chorioamnionitis in the treatment group (p less than 0.05). The period from premature rupture of membranes to delivery (latency) was prolonged with antibiotics (p less than 0.05) and resulted in significant weight gain in the infants in the antibiotic group (p less than 0.0001). These infants also had higher 1- and 5-minute Apgar scores. Clinically suspected sepsis, respiratory distress syndrome, intraventricular hemorrhage, perinatal death rate, and prolonged hospitalization (greater than 30 days) were also increased in the control group.