Tumor specific suppressor T cells (STC) arise in A/J mice as a consequence of inoculation of syngeneic methylcholanthrene induced fibrosarcomas, S1509a and SAI, and elaborate tumor antigen specific suppressor factor(s) (SF). SF which bears determinants encoded by the K end of the H-2 MHC, and STC which express I-J subregion encoded cell surface determinants dampen the primary response and limit the secondary effector reactivity to tumor in vivo. Daily treatment of normal A/J mice with 2 µl of (DBA/2 × 3R)F1 anti-5R (anti-I-J) antisera beginning at the time of a 106 S1509a tumor inocula resulted in a highly significant reduced tumor growth and an abrogation of the capacity of such mice to adoptively transfer suppression. Furthermore, anti-I-J antisera could also limit tumor incidence and appearance when used with smaller tumor inocula. The sera effect was specific and was lost only after absorption with lymphoid cells of the appropriate haplotype.