The concentration of interferon-alpha was measured by a specific two-site immunoradiometric assay in a variety of fluids and tissues collected during human pregnancy. Maternal blood and blood and tissues from non-pregnant adults contained little or no interferon-alpha. Fetal blood, fetal organs, placenta, membranes, amniotic fluid and decidua all contained significant and roughly equivalent amounts ranging from 1.1 to 10 u/ml (or per g of tissue). These findings demonstrate that the fetus and its immediate surroundings are permeated with interferon. It is suggested that this may play a role in regulation of the maternal-fetal graft relation.