A murine B-cell lymphoma bearing the class II major histocompatibility complex molecule I-Ak was cultured with the protein antigen hen egg white lysozyme (HEL). The I-Ak molecules were purified, and their associated peptides were extracted for characterization. Five HEL peptides were identified. Four contained the 10 amino acid residues HEL 52-61 (DYGILQINSR) but were heterogeneous in length and flanking residues. This core sequence is known to confer a high binding affinity for I-Ak. One additional peptide contained the amino acid residues HEL 48-60. These data demonstrate that the HEL epitope containing residues 52-61 is the most abundant HEL epitope presented on the major histocompatibility complex of the antigen-presenting cells and consequently explains its immunodominance.