Major-histocompatibility-complex homozygous partially inbred adults of the ff strain of Xenopus reject transplants of tumor cells of ff strain origin; ff tadpoles do not. Thymectomy, performed 5 days after fertilization, abrogated the adult tumor-rejection response suggesting that in this system tumor rejection is immunologically mediated by T cells. Thymectomy later in larval life did not alter tumor rejection, but it did reduce T-cell numbers. Tolerance to minor-histocompatibility(H) antigens segregating within the ff family, which was induced by grafting adult skin to metamorphosing larvae, did not affect the tumor-rejection capacity of the tolerant adult hosts. This suggests that the ff-2 tumor expresses (a) tumor-specific antigen(s). Immunization of larvae with tumor cells did not induce tolerance to skin grafts transplanted during adult life. Indeed, such grafts were rejected in accelerated fashion, suggesting that memory cells generated in the larvae persist through metamorphosis.