Spleen cells activated by concanavalin A (Con A) and subsequently irradiated with 1500 R were able to suppress the primary in vitro cytotoxic lymphocyte (CL) response of syngeneic splenocytes to H-2 disparate targets. Similarly activated and irradiated BALB/c (H-2d) splenocytes which were cultured with C57BL/6 (H-2b) stimulators for 24 hr developed cytotoxicity for H-2b, but not H-2d, tumor cell targets. This suggested that Con A-activated cells allogeneic to the stimulator might suppress the development of cytotoxic lymphocytes by eliminating antigen early in mixed leukocytes culture. This hypothesis was supported by the observation that suppression of primary cytotoxic responses was overridden by the use of greater numbers of stimulator cells. Moreover, Con A-activated (A X B)F1 lymphocytes suppressed the response of parent A to the H-2 disparate strain C to a much greater extent than they suppressed A-anti-B responses generated in such cultures. Addition of greater numbers of F1 suppressor cells did reveal a suppressive effect not readily explicable on a cytotoxic mechanism, in that modest but significant suppression of the A-anti-B response was observed. Thus, antigen elimination is a major, but not exclusive, mechanism in the suppression of CL responses by Con A-activated cells.