Spleen cells from an SJL mouse immunized with B10.S spleen cells were fused with the nonsecretor myeloma line NS.1. One established hybrid cell line continuously secreted antibody that recognized a new antigenic specificity, tentatively called "Ly-m11." This newly found antigen is detectable on nearly 100 percent of spleen and lymph-node cells, 70 percent of bone-marrow cells, and 20 percent of thymus cells by direct cytotoxicity assays, and on the cells derived from kidney and liver. Strains that are Ly-m11 (+) include C57BL/6, C57BL/10J, B10.S, C57BR/cdJ, C57L/J, and C57BL/KsJ. Other mouse strains so far tested are Ly-m11 (-). The strain distribution pattern distinguished Ly-m11 from any known murine lymphocyte alloantigens, but it follows the H-3 alpha haplotype which is defined by skin transplantation. Linkage tests of nine congenic strains of H-3 and/or H-13/alpha loci and five recombinant inbred lines including CXB, BXH, AKXL, SWXL, and BXD revealed no recombinations between H-3 and Ly-m11 loci on chromosome 2. This newly discovered Ly-m11 alloantigen could itself constitute a minor histocompatibility antigen detectable by serological means.