Although the administration of prophylactic antibodies for intrauterine transfusion is controversial, little information is available regarding placental transfer of antibiotics administered to the mother, or whether the presence of hydrops affects this placental transfer. Sixteen intravascular intrauterine transfusions were performed in 10 patients. Seven (10 procedures) patients were given 2 gm of cefazolin before the procedure and samples were obtained by fetal vascular access. Three patients (six procedures) were given 4 gm of piperacillin and samples were similarly obtained. Specimens were obtained for fetal serum, maternal serum, and amniotic fluid antibiotic concentration. The mean serum cefazolin concentration in hydropic fetuses was 18.04 +/- 3.37 micrograms/ml, and in nonhydropic fetuses the concentration was 21.02 +/- 17.8 micrograms/ml (p = 0.72). The mean fetal serum concentration of piperacillin was 22 +/- 12 micrograms/ml. The placental transfer of both drugs was similar. We conclude that the transplacental passage of these antibiotics is prompt and that the presence of hydrops does not significantly impair the passage of cefazolin.