At 7 days after MRMT-1 inoculation at the thigh of 5 week-old female SD rats, the tumor was treated cryosurgically by 2 cycle freezing, contact method at -170 degrees C. Weights of thymus and the spleen, histological findings of lumbar lymph nodes, PHA-induced blastogenesis of lymphocytes obtained from lumbar lymph nodes or peripheral blood, and resistance rate to tumor re-challenge were examined at 1, 3, 6, 10 and 17 week(s) after cryosurgery. Following results were obtained. (1) Weight of the thymus gradually decreased by 3 weeks after cryosurgery, while that of the spleen increased by 1 week, recovering the preoperative level at 7 weeks. (2) Paracortical hyperplasia of lumbar lymph nodes markedly increased at 1 week and sinus histiocytosis increased after 3 weeks, both keeping their high values till 10 weeks, while germinal center hyperplasia showed a high value at 2 weeks, thereafter decreasing gradually. (3) PHA-induced blastogenesis of lumbar lymph nodes significantly increased at 1 week, keeping its high value till 10 weeks. (4) PHA-induced blastogenesis of peripheral lymphocytes showed the lowest value at 3 weeks, then significantly increasing at 6 weeks. (5) Resistance rate to re-challenge showed the lowest value at 3 week, reading the highest level at 10 weeks after cryosurgery. From the above results, it was suggested that anti-tumor immunity induced by cryosurgery was at the lowest level at 3 weeks, and gradually increased starting at 6 weeks. Augmentation of helper T cell and cytotoxic T cell functions at the regional lymph nodes was also suggested to occur from 1 week to 10 weeks after cryosurgery.