Thymosin alpha 1 is one of several cytokines produced by the thymus that modulates immune function. The presence of elevated serum levels of thymosin alpha 1 in pregnant women and their newborns has suggested that this peptide may play a role in perinatal immunology. In this investigation, we used a microenzyme-linked immunosorbent assay to assay amniotic fluid for immunoreactive thymosin alpha 1 and found levels that were remarkably higher than newborn serum levels (P less than 10(-4]. The increase of thymosin alpha 1 in amniotic fluid with fetal age was natural logarithmic (r = 0.838, P less than 10(-6]. Thymosin alpha 1 in amniotic fluid may account for some of the immunologic properties of this medium.