A porcine genomic clone encoding a major histocompatibility, (MHC) antigen was isolated by direct screening of a swine genomic library with a heterologous human MHC cDNA probe. Mouse L cells transformed with DNA from the clone stably express swine MHC antigen. Pig alloantisera specifically lyse transformant but not control cell lines in a complement-mediated cytotoxicity assay. Direct immunoprecipitation of radiolabeled cellular protein from transformed lines by pig alloantiserum results in the coprecipitation of swine MHC heavy chain and mouse beta 2-microglobulin, demonstrating the association of heterologous subunits of MHC antigens.