To evaluate changes in feto-placental markers with plasma exchange in pregnancy, two patients at varying stages of pregnancy referred to a tertiary care hospital and requiring plasma exchange for intercurrent problems were evaluated. Alpha-fetoprotein, human chorionic gonadotropin, and free estriol were sequentially measured in the patients' plasma and in the fluid removed, thus permitting calculations of permeability rates and clearances. Despite markedly different molecular weights, all three feto-placental markers had similar permeabilities and clearances. While in both patients maternal levels of alpha-fetoprotein and human chorionic gonadotropin decreased rapidly with plasma separation and rebounded rapidly to baseline, free estriol responded differently and did not appear to decrease with therapy. Maternal levels of feto-placental markers only transiently changed with plasma exchange during pregnancy and rapidly returned to baseline with no apparent consequences to the pregnancy.