Endogenous retrovirus-3 (ERV-3) is an endogenous retrovirus encoding an open reading frame for an envelope protein expressed in placenta. In this study we also found high levels of expression in fetal heart, with peak expression occurring between 11 and 17 weeks of gestation. Antibodies to a peptide corresponding to a predicted epitope of ERV-3 were studied by ELISA in sera from 32 healthy women, 47 women during pregnancy, 19 post-partum, 34 with Sjogren's syndrome (SS), 28 with systemic lupus erythematosus (SLE) and 48 mothers of babies with congenital heart block (CHB). Elevated levels of antibodies to ERV-3 were found in normal pregnancy and in patients with SS or SLE. Compared with normal sera the highest levels occurred in mothers of CHB babies (P < 0.001). Antibodies from sera from three CHB mothers bound to recombinant transmembrane protein of ERV-3 on immunoblots, and to sections of fetal cardiac tissue and placenta. This study has shown evidence of autoimmunization to ERV-3 during pregnancy, with particularly high levels of antibodies in mothers of CHB babies. The expression of ERV-3 in fetal heart and the presence of antibodies in maternal sera suggest a possible role in the pathogenesis of CHB.