Fundamental and clinical studies on the perinatal use of cefotaxime (CTX, HR-756), a new cephalosporin antibiotic, were done, with the following results. 1. Following intravenous administration of 2 g of CTX, transport of CTX from maternal serum to umbilical cord serum and to amniotic fluid was found to be good. CTX was not transferred into mother's milk. 2. In clinical use, CTX was given to 16 pregnant patients with premature rupture of the membrane. It showed excellent efficacy in preventing perinatal infection. Five patients with perinatal infections were administered CTX, and in 4 patients CTX had good efficacy. No side effects were noted in any cases.