The secondary cytotoxic responses to the male specific antigen (H-Y) in in mice show H-2 restrictions so that cytotoxic female cells must share K and/or D end antigen with the male target cells. The production of cytotoxic cells is under the control of Ir genes, thus offering the possibility of studying the function of Ir genes in H-2-restricted cytotoxic responses. There are two kinds of Ir genes regulating this response; the dominant gene in the H-2b haplotype and complementary genes in other haplotypes. Now we have been able to map the dominant gene and some of the complementary genes: the dominant genes is in IAb, and in H-2k/H-2d complementation, the Ir genes are in ICk and ICd, and in H-2k/H2s and H-2k/H-2q complementations, at least the H-2k gene is in IC.