Structural studies on rabbit MHC antigens have used monoclonal antibodies directed against a 42,000 m wt molecule that is not associated with beta-2 microglobulin (B2M). Although no structural difference between the molecules associated with or not associated with B2m were revealed by amino terminal sequence analysis or by two dimensional electorpheresis, the B2M determinants and those recognized by the antibody were in distinct subcellular locations as revealed by electronmicroscopy. In continuing studies of rabbit MHC genes using recombinant DNA techniques a full length cDNA copy of a rabbit class I antigens has been cloned and sequenced. Studies on human MHC antigens will utilize cells from families with well characterized crossovers in the MHC region. The HLA-DR antigens of appropriate family members will be analyzed by protein structural techniques and the HLA genes of these families further characterized in collaborative studies by recombinant DNA techniques.