The HLA complex codes for three distinct 44000 dalton molecules associated with beta2 microglobulin--HLA-A, B and C--each with its own multiallelic series of private antigens. The HLA-B molecule is exceptional in that it also carries a diallelic system, Bw4 and Bw6. One of these, Bw4, is often associated with the A-locus specificity A9. This finding has usually been ascribed to linkage disequilibrium between A- and B-locus antigens. We have shown, however, that an epitope called LHe is actually shared by A-locus and B-locus molecules. This epitope is found on all HLA-B molecules bearing the Bw4 determinant and is also found on all HLA-A molecules carrying the A9 (Aw23 and Aw24) or Aw32 specificities. We consider this a "public" HLA antigen; the possible molecular basis for both subtypic and public antigens on a single glycoprotein is discussed.