In the present study the in vitro sensitivity to mitogens (phytohemagglutinin, PHA, concanavalin A, Con A) and purified protein derivative (PPD) antigen of lymphocytes of children exposed in utero to short-term (n = 7) or long-term (over the whole pregnancy, n = 9) action of synthetic glucocorticoids was evaluated. Blood samples from these children and their age- and sex-matched controls were collected 3–4 and 3–10 years after delivery in the short-term and long-term exposure groups, respectively. After short-and long-term exposure to glucocorticoids the counts of all lymphocytes and alpha-naph-thyl-acetate-esterase-positive lymphocytes in the peripheral blood, and the responses of T lymphocytes to mitogens or PPD antigen were similar in the subjects and their controls. The present data thus provide additional evidence of the safety of prenatal glucocorticoid prophylaxis for respiratory distress syndrome. Fetal lymphocytes also seem to be resistant to the extended action of exogenous glucocorticoids.