INTRACELLULAR binding of antigenic peptides by polymorphic class I major histocompatibility complex molecules creates the ligands recognized by receptors of CD8⁺ T cells¹. Previously described in vitro assays of peptide binding to class I molecules have been limited by either the low proportion of accessible binding sites or the lack of allelic specificity^2–6. Here we describe a system in which the human class I molecule HLA-B27 binds considerable amounts of an influenza peptide with precise allelic discrimination. Binding requires viable cells, is stimulated by γ-interferon and is inhibited by brefeldin A. Our results are consistent with the presence of fairly stable 'empty' HLA-B27 molecules at the cell surface. By contrast, analysis of the binding of a second influenza peptide indicates that empty HLA-Aw68 molecules are relatively short-lived. We speculate that HLA-B27 might bind extracellular peptides in vivo and that this property could underlie its association with autoimmune disease.