SUMMARYMonoclonal antibody 212.i.4.2 mediated complement‐dependent lysis of spleen and lymph node cells carrying the tw1, tw12, tw71, t6, tw73, and tLub1 haplotypes, while cells from mice carrying 11 other t haplotypes were not lysed. The antibody also detected an epitope controlled by genes in the H‐2Dd region of non‐t mice. A molecule of 46,000 molecular weight was immunoprecipitated by 212.i.4.2 from detergent extracts of 125I‐labelled spleen cells of +/tw12 and B10.D2 mice. The H‐2dm2 mutation did not alter the expression of the epitope recognized by 212.i.4.2. However, the H‐2dm1 mutation decreased the reactivity of lymphoid cells with the antibody in cytotoxicity tests, and 212.i.4.2 immunoprecipitated little or no protein from extracts of B10.D2(R106) spleen cells which carry the H‐2dm1 mutation.