Pharmacokinetic and clinical studies on imipenem/cilastatin sodium (IPM/CS) in the perinatal period were carried out, and the results obtained are summarized below. 1. IPM/CS transfers to umbilical cord plasma and amniotic fluid were effective. Passage of IPM/CS into mother's milk was as little as with other beta-lactam antibiotics. 2. Among 81 patients with perinatal infections, clinical efficacies were excellent in 16 patients, good in 62, poor in 3, with an efficacy rate of 96.3%. In addition, IPM/CS was effective in 6 patients for prophylaxis. 3. Out of 82 isolates examined for bacterial responses, 69 isolates were eradicated and 8 isolates were decreased. Especially, of 13 isolates of Enterococcus faecalis, 12 were eradicated. 4. In 109 patients administered with IPM/CS, vomiting was observed in 1 patient. No abnormal laboratory test values were observed. 5. IPM/CS was an effective and safe new antibiotic in perinatal infections. We recommend a normal dose of IPM/CS of 0.5 g/0.5 g twice a day and a dose for severe infections of 1 g/1 g twice a day in the perinatal period.