A variety of genetic and biochemical evidence points to an association between major histocompatibility complex (MHC) haplotype and several types of cell surface receptors including epidermal growth factor and insulin receptors. We report evidence for such associations between human class I MHC antigens, HLA antigens, and specific insulin binding sites on human B lymphoblasts. We have measured insulin binding to cells of an HLA-heterozygous, Epstein-Barr virus-transformed B-cell line, LCL 721, and to derivative mutants from which all or part of the HLA complex had been deleted. The affinity, Ka, of insulin binding sites is approximately 10(8) M-1 in mutants expressing antigen HLA-B5 together with other HLA antigens and in mutants expressing only HLA-C. HLA-A1; HLA-A1,B8; HLA-A2,C; and HLA null mutants (not expressing any HLA antigens) bind insulin to sites with an affinity of approximately 10(9) M-1.