With a view to finding a relationship between immune response and MHC in man, 83 D negative mothers with allo-into-D antibodies and 26 PLA1 negative mothers with allo-anti-PLA1 antibodies were investigated as regards their HLA-A, B and DR antigens. We have found that there is a highly significant relation between the DR3 antigen and an immune response to the PLA1 antigen, but none to the D antigen.