Rabbit antibodies to human trophoblast crossreact with human lymphocytes and recognize allotypic determinants on lymphocyte membranes. On the basis of animal and human experiments, it has been suggested that these antibodies may detect an unknown Class I antigen (TLX), encoded by genes within the MHC, and be important in the maintenance of a successful pregnancy. Human antibodies to TLX or TLX-like antigens have not yet been characterized. The aim of this study is to a) isolate and characterize antibodies to TLX-like antigens in pregnancy sera and/or placenta eluates, and b) to study their relationship to antigens encoded by genes within the MHC. To achieve these goals, antibodies in pregnancy sera and placenta eluates will be absorbed and then eluted from trophoblast membranes of 50 individual placentas. The eluates will be tested for the presence of allotypic cytotoxic and/or noncytotoxic antibodies to lymphocytes, T cells and B cells of HLA typed random individuals and families using microcytotoxicity, erythrocyte antibody rosette inhibition (EAI), and ADCC methodology. The eluates will also be tested against placenta sections for their specificity to trophoblast using the indirect immunoperoxidase staining technique. The class of antibody eluted will be determined by Ouchterlony gel diffusion against rabbit anti-human IgG and IgM. Capping experiments will be performed to demonstrate if the eluted antibodies react with HLA or other antigens. In the second set of experiments, 50-100 newborns and their parents will be HLA typed, and trophoblast membranes of each newborn placenta will be used to absorb antibodies from eluates of their own placenta, and from pregnancy serum of their mother. The absorbed antibodies will be eluted and then tested for cytotoxic and noncytotoxic paternal antibodies. Pattern analysis will be performed to demonstrate patterns obtained with different trophoblast eluates and correlation coefficients will be calculated between the reactivity pattern of each eluate and each known HLA antigen. Reactions within families will be analyzed for segregation with a given haplotype. the significance of the study is that these experiments will provide additional information about antibodies to trophoblast membranes and may provide human reagents to TLX-like antigens.