Investigation of the molecular heterogeneity of H-2Db region antigens by using monoclonal antibodies and complex alloantisera has yielded evidence for the existence of only one gene product in this region. Sequential immunoprecipitation and isoelectric focusing analysis suggest that the private H-2.m2 and public H-2.m64 specificities defined by monoclonal or alloantibodies are present on the same molecule. Blocking studies with these reagents indicate these specificities represent two determinants spatially separated on the H-2Db molecule. Functional studies suggest that effector T cells may be generated against both of these determinants or closely associated epitopes, but the majority of the T cells recognize an epitope associated with or identical to the private specificity H-2.m2. Studies of the D region-encoded products of haplotypes besides H-2b have shown two or more distinct molecules can be detected by anti-H-2.m64 in both the H-2d and H-2q haplotypes. The presence of H-2.m64 on a single Db molecule is unique and gives further evidence for the lack of other gene products encoded in this region. Speculations based on the molecular heterogeneity of the D region gene products are made concerning the evolution and genetic organization of H-2 genes.