Human and rabbit antibodies to trophoblast-lymphocyte cross-reactive (TLX) antigens were employed in an enzyme-linked immunosorbent assay (ELISA) to identify and characterize the TLX alloantigen system on human platelets. Neither washing nor extraction in chaotrope or acid altered platelet TLX. The antigen was significantly changed by pronase and trypsin digestion, but Folch extraction yielded antigen in the hydrophilic interface, suggesting carbohydrate. Rabbit antibodies prepared to HLA-negative human syncytiotrophoblast TLX antigens were shown by platelet ELISA to have the same specificity and similar allotypy as anti-TLX antibodies from secondary (2 degrees) spontaneously aborting women. Patients with normal pregnancies before becoming 2 degrees aborters had both IgG and IgM antibodies to TLX. Anti-TLX in patients who never had a normal pregnancy were predominantly IgG. ELISA reactions performed with different concentrations of protein in the buffers detected anti-TLX activity in buffers containing high protein concentrations. This has been observed in studies of blocking antibodies in graft-versus-host disease and immune responses to tumor cells. Platelet TLX offers a new genetic and immunological approach to study similarities of the host-parasite relationships in pregnancy, transplantation, and cancer.