ABSTRACT: The thymus directs T-lymphocyte development and contributes to the maintenance of immune homeostasis, in part, through its production of peptides known as thymosins. In pregnancy, maternal serum levels of thymosin α1 have been reported to be low at midgestation and to increase by term, suggesting that maternal levels represent fetal levels. To evaluate this further, we obtained maternal venous and newborn mixed cord blood from 90 pregnancies between 20 and 42 weeks of gestation at delivery. An ELISA was used for thymosin α assay, and analysis was by paired t test and regression. Maternal and newborn levels were independent of gestational age, but an apparent association (r = 0.51) between the two was inconclusive. Maternal levels (1,207 ± 947 pg/ml) tended to be higher than those of healthy adults (1,043 ± 576 pg/ml). Mixed umbilical cord serum levels (1,466 ± 940 pg/ml) were higher than maternal levels (P ≤ 0.005). Although maternal thymosin α levels may reflect fetal levels, immunological perturbations related to parturition appear to influence both.