The antigenic specificities H-2.4 (private) and H-2.28 (public) in the H-2a haplotype are controlled by the D region of H-2 as defined by the available recombinants. In previous studies we have demonstrated by the antibody-induced redistribution method that the antisera against these specificities contain antibodies against at least two different polypeptide chains. We here report the results of the indirect immunoprecipitation of radiolabeled antigens after solubilization with Nonidet-P40. The antisera against the two specificities precipitated from such extracts two different and independent polypeptide chains, indicating that the products of the D region, as presently defined, comprise at least two different molecules. The molecular weight of both chains is approximately 45 000, which is similar to other molecules bearing private H-2 antigenic specificities. Consequently, the chromosomal segment presently defined by recombination studies as the D region, must contain another locus, controlling the second polypeptide chain which is detectable by anti-H-2.28 antisera, besides the H-2D locus which controls the polypeptide chain bearing the private specificity H-2.4 as well as most of the public specificities.