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PEPTIDES bound to class I molecules are 8–10 amino acids long, and possess a binding motif representative of peptides that bind to a given class I allele^1–4. In the only published study of naturally processed peptides bound to class II molecules (mouse I–A^b and I–E^b), these peptides were longer (13–17 amino acids) and had heterogenous carboxy terminals but precise ammo-terminal truncations⁵. Here we report the characterization of acid-eluted peptides bound to HLA-DR1 by high-performance liquid chromatography, mass spectrometry and microsequencing analyses. The relative molecular masses of the peptides varied between 1,602 and 2,996 (13–25 residues), the most abundant individual M_r values being between 1,700 and 1,800, corresponding to an average peptide length of 15 residues. Complete sequence data were obtained for twenty peptides derived from five epitopes, of which all but one were …