T cells mediating hen egg lysozyme (HEL)-specific delayed hypersensitivity can be activated by synthetic peptides of the 1-18 amino acid residues of hen egg lysozyme. The N-terminal 1-18 peptides of hen egg (PHEL) and ring-necked pheasant lysozyme (PREL) are highly cross-reactive in the induction of T cells mediating delayed hypersensitivity. The N-terminal 1-18 peptides of hen egg and ring-necked pheasant lysozyme (PHEL and PREL) are not cross-reactive in the induction of suppressor T cells, demonstrating that phenylalanine at amino acid residue 3 is critical for the formation of an epitope recognized by T suppressor cells.