DRw53 antigens were isolated from DR7-homozygous and DRw9-homozygous cell lines with an alloantiserum, Hon, containing an anti-DRw53 alloantibody. Partial N-terminal sequence analysis of the isolated DRw53 antigens revealed that DRw53 alpha-chains have sequences indistinguishable from those of DR alpha-chains. In contrast, DRw53 beta-chains have sequences distinct from those of the DR beta-chains and DQ beta-chains isolated from the same cell lines. The DRw53 beta-chains are also distinct from DP beta-chains. These results suggest that the DRw53 beta-chains chains are products of a gene that has not yet been well characterized in the HLA-D region.