Autoimmune thrombocytopenic purpura (ATP) is an antibody-mediated autoimmune disease of platelets. Transplacental passage of the antibody during pregnancy can result in transient neonatal thrombocytopenia, but it is not known why some infants of antibody-positive, thrombocytopenic mothers are not affected. We have developed a competitive enzyme-linked immunosorbent assay (ELISA) to measure circulating antiplatelet antibody and have used this technique to investigate the influence of maternal titers on the occurrence of neonatal thrombocytopenia. The assay is sensitive over a range of 12.5 to 800 ng of immunoglobulin G (IgG) per microtiter well and closely correlates with the complement lysis inhibition assay (CLIA) for antiplatelet antibody (correlation coefficient = 0.726). In many instances, the level of circulating antiplatelet antibody in maternal and cord bloods reflected the degree of maternal and neonatal thrombocytopenia, but several important exceptions were observed. We suggest that levels of antiplatelet antibody in the maternal blood and cord blood are not always predictive of the degree of neonatal thrombocytopenia.