T cells are activated by an interaction of their TCRs with a complex made up of antigenic peptide bound to the interhelical groove of MHC molecules. The helices lining the antigen binding groove of MHC molecules are felt to contribute several contact residues for TCR binding. Peptides derived from the amino acid sequences of these helices may be capable of modulating immune responses and aiding in the dissection of immune recognition. These studies address the effects of a peptide derived from the sequence of amino acids 68-83 of the IAk beta 1 domain (IAk 68-83) predicted to represent a portion of an antigen-binding helix on the IAk molecule. The IAk 68-83 peptide is bound by a monoclonal anti-IAk antibody and inhibits its binding to IAk-bearing cells. The IAk 68-83 peptide inhibits antigen-dependent activation of the IAk+con-albumin restricted T cell clone D10.G4, and this effect is more pronounced at lower doses of antigen-presenting cells. The free peptide has a small effect in limiting binding of anticlonotypic antibodies to D10.G4, and a multivalent form bound to BSA has a more pronounced effect in this regard. The BSA-peptide conjugate, when fluoresceinated, specifically stained D10.G4 cells, and this was specifically competed by unfluoresceinated IAk 68-83 peptide-BSA conjugate, as well as by anticlonotype. These results suggest that peptides derived from the predicted helical region of MHC class II molecules may have a direct interaction with T cell receptors. Such peptides may be capable of modulating immune responses in a physiologically significant manner.