The specificities of B10.A B and T cells responding to pigeon cytochrome c have been examined. Proliferating T cells recognize glutamine 100 and lysine 104 and can be stimulated in vitro by either native cytochrome c molecules or certain of their CNBr-cleavage fragments. In contrast only molecules with the native cytochrome c conformation were found to interact with B10.A antipigeon cytochrome c antibodies. Antibodies appear to recognize a determinant or determinants which overlaps with that which elicits the T cell proliferative response.