The carrier determinant in the N- and C-region of hen egg-white lysozyme (lysozyme), recognized by T cells, which helps antibody formation to a single DNP residue bound to the Lysine-33 of lysozyme, was analysed. P17 (sequence 1--27: Cys 6-Cys 127: 123--129) and P17t (sequence 1-Homoser 12: Cys 6-Cys 127: 123--129), which are known to be immunodominant antigenic determinants could induce helper T cells. When P17t was reduced and alkylated, one of the resultant peptides, P17tN (sequence 1-Homoser 12) could still induce helper T cells, but P17tC (sequence 123--129) could not. A synthetic peptide SP1-14 (sequence 1-Ala 6--14) also induced helper T cells. Thus the carrier determinant helping in antibody formation to the DNP residue at Lysine-33 may be localized in the region corresponding to the sequence 1--12 of lysozyme.