Six regions (T sites) of myoglobin (Mb) were found by a comprehensive synthetic strategy to stimulate Mb-primed lymph-node cells. To define precisely the N-terminal boundary of the immunodominant T site (residues 107-120) with site-specific T-cell clones and to determine the effects of peptide size on their stimulation, two sets of peptides were employed. In one set, the peptides were elongated to the left from His-113 by one-residue increments of the Mb sequence. The other set represented an identical stepwise elongation by one-residue increments of the Mb sequence, but which were extended by additional unrelated ('nonsense') residues to a uniform size of 14 residues. Examination of the proliferative responses of eight T-cell clones, derived from Mb-primed DBA/2 (H-2d) or SJL (H-2s) mice, revealed a dramatic non-specific size requirement. In every clone, the longer nonsense-extended peptides achieved maximum stimulating activity at a lower optimum peptide dose than its natural-sequence, but shorter, analogue. In addition, slight (one-residue) differences in the N-terminal boundaries among the clones was observed. Thus, the fine specificity of each clone was mapped to the region from residue 111 or 112 to about residue 120 of Mb, which coincides with the site of B-cell recognition and resides in a small discrete surface region of the protein chain.