Abstract The results of treatment of pregnant Swiss-Webster mice with various doses of azathioprine are described. Skeletal anomalies, cleft palate, and apparent decrease in thymic size were noted in the fetus when the drug was administered during the embryonic period of development. Equally significant, hydrops fetalis, anemia, and severe hematopoietic depression in the absence of significant maternal hematopoietic depression were observed after administration of the drug in certain phases of fetal development. Fertility of male and female animals was not affected by the dose used, nor was the estrus cycle changed. The possible effects of this drug on the embryo and fetus should be considered whenever it is used for immunosuppression among human females of childbearing age.