Laboratory studies and clinical evaluation of ceftriaxone (CTRX) were carried out with mothers and infants in perinatal period. The presence of synergistic effect between CTRX and amniotic fluid were studied using a broth dilution method. Stronger effects were recognized when both agents were present together compared to each agent alone by the fact that values of MIC and MBC became closer together for Escherichia coli as well as for Streptococcus agalactiae. Against the growth of E. coli, a synergism was observed, but for S. agalactiae, only an additive effect was found. The placental transmission of CTRX upon the administration was rapid, and the blood CTRX level reached its peak shortly after the intravenous administration of the drug. The transport of the drug into the fetus through placenta was excellent and one dose of 1 g of CTRX gave drug concentrations in the umbilical cord serum and amniotic fluid higher than MIC's against main pathogenic organisms. According to these results, it should be possible to treat or prevent perinatal infections by a dose of one gram per day of CTRX, once or twice daily. Cases of perinatal infections were treated with CTRX. An effective treatment without side effects was obtained. No physical abnormalities nor unusual laboratory test results were recognized in neonates delivered from mothers who received CTRX administration. The penetration of CTRX into mothers' milk was low, thus the drug transfer into neonates through the breast-feeding should not be a problem. It appears, from the above study, that CTRX is a clinically useful antibiotic for the prophylaxis and the treatment of perinatal infections.