SUMMARYThe C3H UV‐induced fibrosarcoma, 1591, is highly immunogenic and, therefore, is readily rejected when transplanted into immunocompetent syngeneic recipients. Previous analysis of 1591 with tumour‐specific or H‐2‐reactive monoclonal antibodies revealed that this antigenicity might be due to the expression of two novel class I major histocompatibility complex (MHC) antigens. In this report we describe the molecular cloning and initial characterization of three genes which account for all of the unique serological class I reactivities observed on this tumour. These include two distinct, but highly conserved, H‐2L‐like genes, and a third gene the product of which bears determinants which are characteristic of both the tumour and of class I products of theH‐2khaplotype. Moreover, each of these genes contains a polymorphic restriction enzyme fragment which is detected in the class I sequences of 1591 relative to normal C3H tissue. Since the expression of these polymorphic class I sequences is relevant to the immunogenicity of 1591, the mutational events by which these genes were generated may be significant to the immunobiology o