By using the monoclonal antibody (MAb) c143 against tumor-associated antigen that is expressed in tumor cells of cattle with bovine leukemia virus (BLV)-induced enzootic bovine leukosis (EBL), we found a novel bovine MHC class II-related antigen which consists of alpha chain (36-37 kDa) and beta chain (32 and 34 kDa). The nature of the c143 antigen was different from previously identified class II antigens, such as DR and DQ, as indicated by test for reactivities with mouse L cell transfectants expressing human class II antigens, sequential immunoprecipitation and tryptic peptide mapping. With the progression of EBL, the number of cells carrying the c143 antigen increased, and the beta chain was specifically phosphorylated at serine residue(s) in lymphosarcoma cells and the cell lines derived from them. A marked difference between expression patterns of the c143 antigen and the class II antigens was observed in lymph nodes from BLV-free and -infected animals. Although bovine mixed lymphocyte reaction (MLR) was inhibited by the addition of MAbs against class II antigens, the c143 MAb did not inhibit a lymphoproliferative response of T cells in the MLR, suggesting that the function of this antigen is distinct from those of classical class II molecules. Significantly, although the human homologue of the c143 antigen is expressed little if at all in human T-cell lines, such as CEM, Molt-4, Jurkat and HPB-ALL, its expression become apparent in T-cell lines infected with human T-cell lymphotrophic virus I (HTLV-1).