Suppressor T cells arising during the development of certain murine methylcholanthrene-induced fibrosarcomas have previously been shown capable of limiting only those effector responses generated against the homologous tumor. Thus, S1509a-induced suppressor T cells inhibit immune reactivity only to the S1509a tumor in S1509a immune mice and have no effect on the rejection of SAI tumors in SAI-immune animals. In contrast to this is the cross-reactivity of effector cells in this system, whereby animals rendered immune to either the S1509a or SAI sarcoma are equally capable of rejecting a challenge of the opposite tumor. The specificity of suppression has been further defined in the present study, which demonstrates that S1509a-induced suppressor cells can inhibit responsiveness only to the S1509a sarcoma, even in the simultaneous presence of both the S1509a and SAI tumors. Furthermore, the suppressor factor that is obtainable from suppressor T cells demonstrates a similar precise specificity in its ability to limit selectively reactivity only against the inducing tumor, regardless of the simultaneous expression of antigens on other tumors recognized by cross-reactive effector cells. These results suggest that the antigenic determinants recognized by effector and suppressor T cells are different, and may provide a model for further dissection of suppressor cell function in vivo.