The incidence of tumor metastases was studied in WKA/Hok rats after cryosurgery and surgical excision of primary tumors. When rats with syngeneic fibrosarcoma, KMT-17, were treated by cryosurgery 5 days after transplantation of the tumor, 15 out of 31 rats (48.4%) died with tumor metastases. In contrast only 4 out of 34 (11.8%) died with tumor metastasis after surgical excision. Because tumor cells either in the regional lymph nodes or in the peripheral blood were already detectable in about half of the rats treated by either cryosurgery or surgical excision, mechanical and physical control of enhanced metastasis by cryosurgery is ruled out. The development of anti-tumor immunity was delayed in the rats treated by cryosurgery as compared with that in rats treated by surgical excision. This was investigated by means of Winn's assay. A marked reduction of tumor metastasis was seen after the surgical removal of cryonecrotized tumor tissue, while implantations of cryonecrotized tumor tissue into the rats treated by surgical excision resulted in an increment of metastasis. The mechanism of the enhanced metastasis observed in this experimental investigation is discussed from an immunological point of view.