Molecular association between major histocompatibility complex (MHC) antigens and cellular proteins are thought to be involved in various immunological and nonimmunological functions of MHC antigens, including hormone signaling. The existence of physical interactions between insulin receptors and MHC class I antigens was investigated in liver plasma membranes from congenic H-2k mice. Insulin receptors were specifically labeled with a 125I-labeled photoreactive insulin analogue, and cellular proteins were solubilized and incubated with various monoclonal antibodies. Immunoprecipitates were analyzed by polyacrylamide gel electrophoresis followed by autoradiography. Antibodies reacting with distinct epitopes on H-2k class I antigens were all able to precipitate up to 25% of the labeled insulin receptors in H-2k mouse liver membranes, whereas no insulin receptors were precipitated in H-2b mouse liver membranes. Sequential immunoprecipitations showed that insulin receptors and H-2 antigens were coprecipitated and that no cross-reactivity occurred. The specificity of the interaction between insulin receptors and H-2 antigens was demonstrated after double labeling of membrane proteins by photoreactive insulin and lactoperoxidase-catalyzed iodination. These results thus show that, in mouse liver membranes, insulin receptors are physically associated to class I antigens of the MHC.