Monoclonal antibodies have been used to isolate and characterize human HLA-DR antigens from human B cell lines. Using sequential immunodepletion experiments, several subsets of DR antigens have been identified. Partial amino-terminal amino acid sequencing has identified these subsets as DR molecules with sequences homologous to the murine I-E alloantigens. In addition, sequence data provide evidence for primary structural differences in the DR beta-chains of two of the subsets. These results provide primary structural evidence for the presence of multiple DR beta-chain loci.