Studies on radiolabeled, detergent-solubilized H-2 alloantigens of two different genotypes with indirect immunoprecipitation show that at least two different antigen molecules can be isolated from cells of each genotype. These H-2 alloantigen molecules may bear the antigenic sites for one, two or more specificities. The findings are consistent with recent views that the H-2 genetic region includes two genes, each of which directs the synthesis of a different H-2 alloantigen molecule.