Many factors during fetal life and early infancy have been found to affect the development of immune responses in animals. This study investigated whether acute exposure of the fetal monkey to high levels of corticosteroids would also have a lingering effect on the expression of immune responses still manifest postpartum in yearling juveniles. One month prior to parturition, pregnant rhesus monkeys were administered dexamethasone for two days. Lymphocyte proliferative responses to mitogen were then examined in their offspring when they were between 1.0-1.5 years of age. In addition, cell sensitivity to corticosteroid feedback was assessed by testing the ability of a gradation of cortisol doses to inhibit proliferation. Monkeys generated from dexamethasone-treated pregnancies tended to have lower responses to concanavalin A. Further, their cells were less sensitive to in vitro incubation with cortisol, suggesting that elevated adrenal activity in vivo had downregulated hormone receptors on their cells. These findings concur with the view that steroidal hormones in utero can influence the fetal immune system, resulting in prolonged effects on immune responses after birth. The similarity of the dexamethasone condition to the clinical treatment used in obstetrical practice raises a potential concern about the widespread antenatal exposure of premature infants to steroidal drugs.