Horse and tuna cytochrome c synthetic peptides 39-53 can equally well stimulate a cytochrome c specific T cell hybridoma. The amino acid sequence of the two peptides differs mainly for the presence of a helix-breaking proline and a helix-forming glutamic acid in position 44. Circular dichroism studies of these two peptides revealed that their conformation could be drastically depending on the solvent used. These data place some limits on the correlation between structural data and biological activity.