Tumor isoimmunity has been questionable; thus, two experiments were carried out using mice of isogenic lines (C(3)H×dd: F(1)). Experiments were with tumor-bearing mice treated with anticancer drugs, and with mice immunized with heavily irradiated tumor cells (MH 134). 1. By treating the tumor-bearing mice with anticancer drugs, the γ-Globulin titre increased in the subject serum, followed by a marked decrease as the tumor increased. With anaerobic glycolysis and with neutralization test of tumor cells, the inhibitory action of sera were ineffective. 2. The result of x-ray irradiation on MH 134 tumor cells in vitro, produced total stain using Eosin when irradiated with more than 8000r. With the same dose of irradiation, tumor cells lost their transplantability in sensitive F(1) mice. 3. F(1) mice, immunized with heavily irradiated MH 134 tumor cells which lost their transplantabilty, became resistant to the transplantion of relatively few, fresh MH 134 tumor cells. 4. F(1) mice immunized with heavily irradiated MH 134 tumor cells, were bled and their organs excised. These sera, organ homogenates and organ suspensions on being tested for anticancer activity, revealed greatest antitumor activity in the spleen followed by serum and liver.