The HLA-Bw4 and Bw6 antigenic determinants have been shown to co-migrate with HLA-B determinants on gel-filtration, sucrose density gradient centrifugation and affinity chromatography, using Lens culinaris lectin and antibody against human β2 microbulin. These and other published data imply that the HLA-Bw4 and Bw6 determinants reside on the same polypeptide chain as other HLA-B locus determinants. The implications of this in terms of the evolution of cross reacting groups of antigens at the HLA-B locus are discussed.