Radioactivity was recovered in fetuses of pregnant rats following administration of radioactive T2-toxin. The transplacental effects of T2-toxin were studied by ip administration or oral feeding of pregnant rats at doses equivalent to natural contaminations and compatible with the maintenance of pregnancy. Body weights of the newborn rats from treated females were similar to the body weights of the control animals but their thymuses were atrophic. This atrophy was reversible in one week. Since the measurement of antibody production for fetuses and newborns is not feasible, the lymphoblastic response to mitogen of the splenic and thymic cells of baby rats from treated and control females was tested. At 4 and 6 days after birth, a good response to PHA for the thymic cells of the mother treated young rats was observed. Histological examination of the thymus showed that one day after birth the cortex was atrophic while the medulla was proliferative; on day six the situation was reversed. For the spleen, both B and T cells were impaired and their responsiveness to PHA and LPS decreased. On days 1 and 6, the periarteriolar sheats, as well as the follicles, appeared atrophic. These results show that T2-toxin easily passes the placental barrier and that T2-toxin injection or feedings at levels (2 ppm) similar to those in naturally contaminated foods, produced an impairment of the immune system of the newborn.