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The mouse major transplantation antigens H–2K, H–2D and H–2L are highly polymorphic cell-surface glycoproteins which may serve as recognition elements in cell–cell interactions1. Each antigen possesses a number of alloantigenic determinants defined by antisera of various specificities. Recently, monoclonal antibodies have been produced which redefine and extend our knowledge of these determinants2,3, but structural information has not yet been correlated with the serological definition of the antigens. We have previously reported the molecular cloning of genes for H–2Ld and H–2Dd transplantation antigens from the BALB/c mouse and the expression of these genes in mouse L cells4,5. To localize the serological determinants to discrete regions of the H–2 protein, we have now constructed new H–2 antigen genes by joining together fragments of the H–2Ld and H–2Dd genes. In L cells, these genes direct the synthesis of hybrid H–2 proteins and by using monoclonal antibodies of defined specificities, we have mapped classically defined serological specificities to structurally defined domains of the transplantation antigen protein. We conclude that polymorphic determinants recognized by monoclonal antibodies are located in functionally distinct portions of the protein.