The ability to modify T cell responses was analyzed using synthetic peptide analogues of the T cell determinant for pigeon cytochrome c. Although the B10.A T cell proliferative response is directed to residues 95-104, residues to the amino-terminal side of this determinant influence antigen-specific T cell recognition. The proposed role of this non-determinant leader sequence has been to stabilize the core determinant in a helical conformation. Previous studies from our laboratory, however, using non-native leader sequences, have demonstrated that factors other than conformational stabilization are responsible for the modulated recognition of the core determinant. In the studies presented here, we investigated the role of the leader sequence using synthetic analogues that contained the core determinant, residues 95-104, and various seven residue non-native leader sequences that were designed to examine the changes to T cell recognition invoked when the determinant was made more or less helical, amphipathic, or lipid binding in character than the native determinant. The structure of each analogue in aqueous, non-polar (TFE) and lipid environments was determined by circular dichroism. The ability of each antigen analogue to bind to phospholipid membranes and to stimulate two different pigeon cytochrome c T cell hybridomas, 2B4 and 22.D11, was also investigated. Our findings suggest that neither helicity or amphipathicity are necessary features of T cell recognition but that electrostatic interactions involving either the lipid membrane or the I-E k molecule may influence T cell stimulation. © 1989.