The monoclonal antibodies detecting the lymphocyte surface alloantigen (Ly-35.1) of wild mouse-derived inbred strains were first reported by Ikegami and colleagues (1988). We now add one more antibody that defines the Mus musculus molossinus-specific lymphocyte alloantigen Ly-36, The monoclonal antibody (IgG2a, K) was secreted from hybridoma clone C12.274, which was obtained by a fusion of spleen cells from the BALB/c mouse hyperimmunized with (BALB/cxMOLF/Ei)F 1 thymocytes and SP2/0 myeloma cells. Reactivity of the antibody was examined by the direct cytotoxicity assay using rabbit serum as a complement source (Tada et ai. 1981). The Ly-36.1-specific antibody reacted with 95%, 70%, 60%, 30%, and 35% of cells in thymus, spleen, lymph node, bone marrow, and peripheral blood, respectively (Table 1). The expression of the Ly-36 alloantigen on kidney and liver, and not on brain, was also determined by the cytotoxicity absorption assays using nonlymphoid tissue homogenates (data not shown, Kimura et al. 1980). Mouse strain distribution of the Ly-36 alloantigen is shown in Table 2. All of the 66 mouse strains, including mouse strains derived from wild mice, were purchased from The Jackson Laboratory, Bar Harbor, Maine. All four strains originated from M. m. molossinus; i.e., MOLC/Rk, MOLD/Rk, MOLE/Rk, and MOLF/Ei were positively typed by the monoclonal antibody C12.274 (Ikegami et al. 1988). Sixty-two other mouse strains, including 11 strains of wild mouse origin, were typed as Ly-36.2. It should be noted that the monoclonal antibody can discriminate the subspecies M. m. molossinus from others, i.e., M. m. domesticus, including PERA/Rk, SF/CamEi, SK/CamEi, and other laboratory mice, and M. m. castaneus (CAST/El). There are two other subspecies of Mus musculus (Potter 1986), M. m. musculus and M. m. bactrianus, which we have not classified yet using Ly-35.1-specific or Ly-36.1-specific monoclonal antibodies. Thorough examination of wild mouse populations