Mutation M523 H-2(ka)) occurred spontaneously in strain CBA/CaLacSto and was discovered during routine skin graft testing from genetic homogeneity. By linkage and complementation tests, the mutation has previously been mapped in the K end of the H-2 complex. It was demonstrated that the mutation occurred in the K region, without affecting the I region in the K end of the complex. The mutant antigens cause rejection of skin grafts, stimulate cells in mixed lymphocyte culture, and function as stimulators as well as targets in cell mediated lymphocytotoxicity. Yet, they are serologically indistinguishable from the antigens of the original strain and do not induce formation of humoral antibodies upon immunization of the CBA strain. Together with the results obtained on testing of other H-2 mutants, the data strongly support the notion that classical H-2 antigens (i.e., products of the H-2K and H-2D loci) can function as lymphocyte stimulating determinants, and that I-region differences are not required for the induction of strong cell mediated lymphocytotoxicity.