Immune spleen cells (ISC) capable of inhibiting the growth of adenovirus type 12 (AdI2) tumors were raised in C57BL/6 mice by immunization with Ad12, fractionated according to their affinity for plastic and nylon-wool substrates or treated with various antisera plus complement, and subjected to the tumorneutralization test (Winn) to define the effector cells for the cell species. Full antitumor activity of ISC was recovered in the cell fractions nonadherent to the two substrates; the antitumor activity of ISC was abrogated entirely by antiThy-I,2 serum and almost entirely by anti-Lyt-2.2 ascites fluid plus complement. These results clearly indicate that T-Iymphocytes, particularly those bearing Lyt-2.2 antigen, are the principal effectors in ISC against Adl2 tumors in animals.