The fine specificity of the response of T cell clones derived from B10.BR and B10.S congenic mouse strains restricted by I-Ak and I-As molecules, respectively, and which recognize the same 17 amino acid sequence (102-118) of myoglobin, has been investigated and compared with that of T cell clones specific for the same peptide with I-A.d The critical amino acid residues within the 102-118 region of myoglobin required for stimulation of I-Ak-and I-As-restricted T cell clones specific for this determinant were compared using a panel of synthetic peptide analogs. Residues 109, 113, and 116 were critical for stimulation of clones from both haplotypes, although the precise fine specificity varied, even among clones using the same restriction molecule. Residues 109 and 116 are also critical for stimulation of myoglobin-specific I-Ad-restricted clones (Berkower, I., L. A. Matis, G.K. Buckenmeyer, F.R. N. Gurd, D. L. Longo, and J. A. Berzofsky. J. Immunol. 132:1370, 1984). There was also considerable overlap in the size of the minimal determinant necessary for full activity: 106-118 for B10.BR and B10.D2 (Cease, K. B., I. Berkower, J. York-Jolley, and J. A. Berzofsky. J. Exp. Med. 164:1779, 1986) clones and 102-117 for B10.S clones. Despite this similarity in fine specificity, T cell clones were genetically restricted and could not be stimulated with the 102-118 peptide presented by Ia molecules of other haplotypes that could also present this epitope to syngeneic clones. These results suggest that binding of an immunogenic peptide to class II molecules is not sufficient to ensure recognition by a given T cell antigen receptor specific for the peptide, but do not indicate whether the major histocompatibility complex molecules interact directly with the T cell antigen receptor or induce a different recognizable conformation of the peptide.