A (B10 × D2.GD)F1 anti-B10.D2 antiserum was prepared in order to examine Ia antigens determined by genes to the right of the I-A subregion of the H-2d haplotype. This antiserum complement-mediated lysis of H-2a and H-2d spleen cells to a titer of 1:256, indicative of its activity against Dd antigens. Absorption with B10.A spleen cells left anti-Ia (B cell) cytotoxic reactivity to a titer of 1:64 when tested on H-2d spleen cells. This correlates with, and helps to define, the recently described I-E subregion specificity, Ia.23, of the H-2d haplotype (C. S. David, in press). Absorption of this antiserum with B10.BR spleen cells remove all cytotoxic activity against B10.A(2R) spleen cells, indicating either that the serum contained no detectable antibodies to private Ia antigens determined by the I-Cd subregion or that the I-C subregion of H-2a is not identical to that of H-2d.