Aztreonam (AZT) was administered to 73 cases and its usefulness and safety were evaluated upon review of the transfer of the drug into maternal blood, umbilical cord blood and amniotic fluid. The result obtained are summarized as follows. 1. AZT was intravenously administered by single injection to obtain actual data. Based on the data, theoretical changes of drug concentrations in serum in mother were reviewed according to the lapse of time. Drug concentration in maternal blood showed its peak immediately after administration and gradually decreased thereafter. Umbilical cord blood concentration showed a lower peak, which was reached with a slight delay to the Tmax for maternal blood concentration, then decreased. The decrease, however, was more moderate than the decrease in maternal blood concentration and, according to the theoretical values, umbilical cord blood concentration was higher than maternal blood concentration at 2.23 or 2.24 hours after administration. AZT concentrations in amniotic fluid slowly increased after administration and were higher than umbilical cord blood concentrations at 1.68, 1.73 hours after administration, higher than maternal blood concentrations at 1.82, 1.85 hours after administration, and they reached their peak somewhat later. Subsequently, high concentrations of AZT in amniotic fluid were maintained for a long time. The result suggests that AZT is useful for prophylaxis and treatment of amniotic fluid infections. 2. Theoretical values were analyzed by applying two-compartment model and three-compartment model to the actually-measured values, and each parameter was compared. T 1/2 of AZT concentrations in maternal blood and in umbilical cord were 1.29 hours, 1.29 hours, 2.14 hours, 2.00 hours; Cmax 187.09 micrograms/ml, 184.15 micrograms/ml, 30.63 micrograms/ml, 30.66 micrograms/ml and AUC 153.25 micrograms.hr/ml, 153.40 micrograms.hr/ml, 123.19 micrograms.hr/ml, 123.09 micrograms.hr/ml respectively showing approximate values. Cmax values of AZT in amniotic fluid were 47.08 micrograms/ml, 47.74 micrograms/ml; AUC 948.03 micrograms.hr/ml, 1,028.70 micrograms.hr/ml also showing approximate values. However, volume of distribution of umbilical cord blood and amniotic fluid showed a difference according to compartment model. 3. It was considered from the above results that application of only two-compartment open model would make analysis possible when only T 1/2, Cmax and AUC values must be measured for mother-to-fetal transfer of a drug. 4. No subjective and objective side effects nor abnormal laboratory values were observed in any of these cases (both mothers and fetuses).