The peptide motifs of HLA-B* 2702 and B'2705 molecules were determined by pool sequencing of natural ligand mixtures, using established methods (Fa! k et al. 1991). B'2702 molecules were immunoprecipitated from C 1R cells transfected with the B'2702 gene (a gift from P. Cresswell), using either W6/32 antibodies (specific for HLA-A, B, C; Barnstable et al. 1978) or TM5 antibodies, specific for HLA-B27 (Thurau et al. 1989). As a source of B'2705 molecules, the following cells were used: LG2, expressing HLA-B* 2705 (Jardetzky et al. 1991); C1R cells transfected with B'2705 (a gift from P. Cresswell); or a mouse (BALB/c)-derived B-cell lymphoma, X63, transfected with the B'2705 gene (a gift from E. Weiss). Antibodies used for precipitation of B* 2705 molecules were PA2. 6 (Parham and B odmer 1978), or W6/32 (both recognizing HLA-A, B, C), or TM5. In some instances, LG2 lysates were precleared with BB7. 2 antibodies (Parham and Brodsky 1981) to remove A2 molecules also expressed by LG2 cells. Ligands were extracted with trifluoroacetic acid; ligand separation was done either on a 4.0 mm or on a 2.1 mm high pressure liquid chromatography reversed phase column using Pharmacia (Freiburg, Germany) equipment; the ligand mixture was sequenced on a model 477/120 (Applied Biosystems, Weiterstadt, Germany), all as described (Falk et al. 1991). Individual ligands were sequenced on a model 476 sequencer (Applied Biosystems).