Pharmacokinetic, bacteriological and clinical studies of imipenem/cilastatin sodium (IPM/CS) were carried out in the perinatal period, and the results obtained are summarized as follows: Effects of IPM on bacterial growth curves of Staphylococcus aureus (sensitive and resistant strains) and Escherichia coli (sensitive strains) in amniotic fluid were determined. The bactericidal effects of IPM increased in amniotic fluid and remarkable increases against resistant strains were demonstrated. IPM and CS were detected promptly after intravenous drip infusion to pregnant women, and reached peak levels shortly after administration in maternal plasma. Placental penetration of IPM and CS to the fetus was favorable. After intravenous drip infusion of 500 mg/500 mg of IPM/CS, drug concentrations in the umbilical cord plasma and the amniotic fluid exceeded MICs against main pathogenic organisms. Clinically, IPM/CS was effective in the treatment of perinatal infections without any side effect. The above results demonstrated that IPM/CS is a clinically useful antibiotic for the prophylaxis and the treatment of perinatal infections.