The processing and presentation by macrophages of the well-defined protein hen egg-white lysozyme (HEL) was analyzed using two HEL-specific T cell hybridomas. The processing studies revealed that both clones required that native HEL be processed, while neither clone required any processing of a, tryptic digest of lysozyme. A differential requirement for processing was found for the intact, denatured lysozyme (CM-HEL) with one clone (2A11) requiring processing, and a second clone (3A9) did not require any processing. The determinant on the HEL molecule that both clones recognized was localized to a tryptic fragment containing residues 46 to 61. By testing the immunogenicity of fragments of the 46-61 peptide, mouse lysozyme, and human lysozyme, we were able to localize the T cell determinant to either of two residues, Gly-49 or Leu-56.