An anti-idiotype serum was raised in a chimpanzee (A) by immunization with autologous lymphocytes primed in vitro against an unrelated chimp (B). This autoantiserum in the presence of complement was cytotoxic for 5 to 7% of the resting lymphocytes from chimp A and for 30 to 45% of the mixed lymphocyte culture (MLC) primed cells (A X Bx), but was not reactive against the lymphocytes of the priming chimp (B). Anti-idiotype antibody and complement treatment of autologous resting or primed cells blocked the ability of these cells to respond in MLC or primed lymphocyte test (PLT) to the stimulator cells from chimp B, but not to cells from a third chimp. When cells from the immunized animal (A) were incubated with the autologous antiserum in vitro, they were stimulated, thus producing cells which had the same activity in PLT as did cells primed against stimulator cells of chimp B. Thus, an autoanti-idiotype serum has been raised in a primate system which identifies the recognition structure on autologous T cells directed against antigeneic determinants on the stimulator cells of a histoincompatible donor.