Spleen cells from BALB/c mice which either bore syngeneic sarcomas or were normal controls were cultured in vitro. The culture supernatants of spleen cells from tumor-bearing mice inhibited (blocked) specific cell-mediated cytotoxicity to the tumor borne by the spleen donor. They also mediated specific antiserum-dependent cytotoxicity with control lymphoid cells. The appearance of blocking activity in culture supernatants was prevented by lysis of theta-positive spleen cells with antiserum and complement. The blocking activity was removed by passing culture supernatants through an anti-mouse immunoglobulin affinity column and was recovered in a 3 M NaSCN eluate of the column.