Conditions are described for diazonium linkage of the hapten p-azophenylarsonate (Ar) to mouse lymphocytes for the purpose of induction of syngeneic cytolytic T lymphocytes. These cytolytic effector cells do not lyse target cells that are coupled with an unrelated hapten, trinitrophenyl. Cell lysis can be blocked by the addition of anti-Ar antiserum. Only those target cells that share H-2 K or D end antigens with the stimulating cell population are susceptible to lysis. Several methods were used in an attempt to demonstrate the presence of the Ar cross-reactive idiotype on CTL induced in A/J mice.