The pharmacokinetics of ceftriaxone (CTRX) and its clinical efficacy in perinatal infections were studied. The obtained results are summarized as follows. 1. Concentrations of CTRX in maternal serum, umbilical cord serum and amniotic fluid were determined following intravenous injection with 1 g of CTRX. Maternal serum levels were not lower than 100 micrograms/ml immediately after administration, and gradually decreased to about 10 micrograms/ml in 12 hours, and to 4 micrograms/ml in 24 hours. The half-life of CTRX in maternal serum was 5.6 hours. CTRX levels in umbilical cord serum were about 7 micrograms/ml at 10 minutes after injection, increasing to 12 to 13 micrograms/ml in 12 hours and decreasing to 5 micrograms/ml in 24 hours. CTRX levels in amniotic fluid were slightly lower than those in the umbilical cord serum, and about 2 micrograms/ml at 10 minutes after injection, and they remained at 4 to 8 micrograms/ml thereafter for 28 hours. 2. CTRX (1 g) was intravenously administered twice daily to 9 patients with perinatal infections for 3 to 7 days. Clinical efficacies of CTRX were judged excellent in 2 cases and good in 7, suggesting that CTRX was effective in all cases. No side effects or laboratory abnormalities were observed in any case. As a result of these findings, CTRX may be considered a very useful antibiotic in perinatal infections.