Syngeneically-pregnant CBA mice were stressed by injecting them with saline, sc twice daily, on days 10-18 of their pregnancy. Pups born to these mothers exhibited disturbances in their immuno-regulatory cell development. Splenocytes harvested from these pups, when one day old, failed to suppress xenogeneic Graft-versus-Host reaction and proliferative response of syngeneic mature cells to concanavalin-A, as control splenocytes did. Also, thymocytes of 4-6-day-old mice, born to stressed mothers, did not exert an inhibitory effect on neonatal suppression, as control thymocytes did. These results demonstrate that neonatal stress is an useful physiological model for studying interactions between some regulatory systems of the organism, in its early stages of development.