Cloned B-cell lines from a female T16H/XSxr mouse in which Tdy expression was suppressed due to X inactivation and from a male X/XSxr mouse, both of the (kxb)F1 haplotype, were examined for H-Y expression. This was determined both by their ability to act as targets for H-2k and H-2b-restricted H-Y-specific cytotoxic T cells and by their ability to stimulate the proliferation of H-2Kk, H-2Db (class I) and Ab (class II)-restricted T-cell clones. In B-cell clones from the T16H/XSxr mouse, expression of H-Y/Db exhibited partial X inactivation and only a proportion (congruent to 30%) of the cells were targets for or stimulated H-2Db-restricted H-Y-specific T cells. In contrast, H-Y epitopes restricted by H-2k (H-Y/Kk, H-Y/Dk) and Ab (H-Y/Ab) exhibited no X inactivation. Furthermore, no inactivation of H-Y/Db, H-Y/Ab, or H-Yk was observed in the male X/XSxr mouse. These results indicate that the T16H/XSxr female is a mosaic, as a result of the variable spread of X inactivation into the Sxr region. They further suggest that the H-Y antigen recognized in association with H-2k and H-2Db class I molecules and Ab class II molecules may be the product of more than one gene.