Murine L cells expressing HLA-A2 or -B7 antigens were isolated after cotransformation of thymidine kinase-negative cells with the herpes simplex virus thymidine kinase gene and the genomic clones containing either the HLA-A2 or -B7 genes. Monoclonal antibody binding analyses demonstrated the stable cell surface expression of HLA antigens by these cells at levels of up to 40% of the amount expressed by the human B lymphoblastoid cell line, JY. The HLA-A2 and -B7 antigens expressed by the L cells retained all of the antibody-defined, heavy-chain-associated antigenic determinants but lacked those determinants associated with human beta 2-microglobulin. These HLA transformants were capable of functioning as targets for monoclonal cytotoxic T lymphocytes (CTL) that specifically recognize the HLA-B7 or -A2 antigens expressed by JY cells. However, the efficiency of lysis, relative to the JY cell line, was 50-99% for individual CTL. In addition, not all of these CTL were capable of lysing the appropriate transformants. Because the antigens appear by serological criteria to be structurally intact and expressed at high levels, these results suggest that the complementation of the HLA heavy chains with mouse, rather than human, beta 2-microglobulin may alter the antigenic determinants that are important for CTL recognition.