The survival of tumor cells at 1,4, and 24 h after single, double, and triple freezing and thawing cycles has been examined. After freezing two xenografted tumors of human origin (GW-77 and GW-127) in the cheek pouch of unconditioned Syrian hamsters with liquid nitrogen under pressure (––196°C), it was found that retransplantation of the cryogenic tumors at 1 h post single freezing and thawing resulted in tumor growth. Intact tumor cells could also be demonstrated microscopically at this time. No viable tumor cells could be found at 4 h or later post single freezing and thawing, or at 1 h or later after multiple freezing and thawing cycles. The role of the cryogenic environment on the survival of viable tumor cells within the frozen lesion, as well as the importance of multiple rapid freezing and slow thawing cycles, followed by a period of delay prior to excision of the frozen tumor, have been stressed.