The immune status of peripheral blood mononuclear cells (PBMC) and sera of pregnant women infected with the helminth Schistosoma mansoni was studied during pregnancy and the cord blood mononuclear cells (CBMC) and sera from their offspring were studied at parturition. PBMC pokeweed mitogen-induced responses were maintained in gravid women, but the responses to both schistosome and non-schistosome antigenic preparations declined progressively during pregnancy. Schistosomal antigens stimulated proliferative responses by the CBMC of many neonates born of infected mothers, but not those of uninfected mothers. These specific responses by CBMC of only neonates born of infected mothers are indicative of in utero, cell-mediated sensitization of the neonates, which could be due either to circulating schistosomal antigens or to anti-idiotypic antibodies which cross the placenta during gestation. Sera from infected mothers and the cord blood sera from their babies showed the same levels of specific IgG anti-schistosomal activity. Anti-schistosomal IgM levels were maintained during pregnancy to some antigens and not to others, while such antibodies were rarely found in cord blood sera, and then only at very low levels.