The present study assessed central nervous system (CNS) and immune system changes in murine fetuses after cyclophosphamide (CP) exposure during intrauterine life. A single CP dose of 0, 10, or 20mg/kg body weight was administered by intraperitoneal inj ection to pregnant mice (20/group) on day 11 of gestation (GD 11) and fetuses were evaluated on day 19 of gestation (GD 19). Fetuses were examined for external changes, and then the brains and thymuses were removed for further evaluations of histological changes, protein content, apoptotic cell count, DNA fragmentation, and in vitro cell proliferation using 1 fetus/litter for each assessment. Brains and thymuses from CP-exposed fetuses were smaller in size and distorted in overall shape compared to those from the control group. Estimated mean protein content (mg/mL) of brains was decreased in the CP-exposed groups. In both brain cells and thymocytes there was an increase in mean apoptotic cell counts and in mean percent DNA fragmentation in the exposed groups. The in vitro cell proliferation assays conducted with cells from exposed fetuses exhibited a mean decrease in the number of both brain cells and thymocytes generated. These findings indicate that maternal CP treatment on GD 11 in mice results in marked fetal toxicity characterized by reduced live litter size, fetal body weights as well as brain and thymic weights and malformations which are accompanied by changes in brain protein content, brain and thymic apoptosis, DNA fragmentation and in vitro cell proliferation at term