The major histocompatibility complex of the rat (RTl) is composed of at least five subregions. They are RTl-A,B,C,D and E regions. RTl-A,E and RTl-B,D regions encode class I and class II alloantigens, respectively. The RTl-C region encodes antigens which are similar to class I alloantigens and they are the homologue of mouse Qa-Tla antigens. A monoclonal antibody (X81-5C9) was produced against a rat B-cell leukemia, KNL-14. The KNL-14 cells were injected to a congenic rat, WKA. 1A(ACI). Spleen cells taken from the congenic rat were hybridized with mouse myeloma cell line P3.X63.Ag8.653. The monoclonal antibody lysed over 80% of nylon wool (N. W) adherent cells of lymph nodes, 25-30% of unseparated lymph node cells and/or spleen cells, and approximately 15-20% of peripheral blood lymphocytes of WKAH strain of rats. Only a portion (30-40%) of N. W. adherent cells of the peripheral blood lymphocytes was killed. Bone marrow cells, thymus cells and N. W. nonadherent cells were not lysed by the antibody. Macrophages, fetus, thymus and kidney homogenates could absorb the reactivity, whereas RBC, epidermal cells, brain, liver, testis could not absorb the cytotoxicity. A survey for the strain distribution of the antigen disclosed positive strains such as WKAH, W/Hok, LEJ, WKA. 1J (LEJ), ALB, WKA. 1B (ALB), BUF, BN, LEW, F344, W. 1L (F344) and KYN rats. The negative strains were NIG-III, WF, WKA. 1U WF), SDJ, W. 1U (SDJ), TO, W. 1T(TO), ACI, WKA. 1A(ACI), BDIX, WKA. 1DV1 (BDIX) and PVG/c rats. Immunochemically, the antibody precipitated antigens that gave two major bands on the SDS-PAGE. The heavy chain has an apparent molecular weight of 30 KD, which shifted to 33 KD under reducing conditions. The light chain has an apparent molecular weight of 12 KD. From these data a monoclonal antibody X81-5C9 is considered to detect a rat MHC (RTl) gene product which is different from the classic class I or class II cell surface antigens. It may be one of RTl-C antigens expressed mainly on B-cells. The characteristic feature of rat RTl-C antigens were discussed in relation to mouse Qa-Tla antigens.