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Surgical removal of the thymus of newborn mice is usually associated with "wasting," with a marked deficiency in the lymphocyte population of the blood, spleen and other lymphoid tissues, and with immunologic deficits, particularly of cell-mediated responses such as the immune reactions of homograft rejection and delayed hypersensitivity reactions.1 The results of thymectomy performed at three to seven days, however, are modified to the extent that "wasting" does not occur and there is a minimal lymphocyte depletion, but the animal is still deficient in its ability to reject allogeneic grafts of normal or neoplastic tissues especially when antigenic differences are slight, for example, if donor aild recipient differ in their makeup of weak histocompatibility genes.'-' Thymectomy in the neonatal period also results in an increased suceptibility to the oncogenic effects of polyoma virus. Such observations have been recorded for the mouse,2' 4, 5 rat,6' 7 and the Syrian hamster.8 In this study we have taken advantage of the animal with a deficient immune mechanism in order to identify the factors of host resistance to polyoma virusinduced neoplasms. C57BL/Ka strain mice remain free of polyoma-type neoplasms, even if virus is given in high concentration at birth.', This resistance, which is genetically determined,3 is broken by neonatal thymectomy. Seventy to 90 per cent of C57BL mice thymectomized at 3 days of age develop neoplasms, principally of the salivary glands, if polyoma virus is given up to 10 days of age and some sensitivity to respond is retained up to at least 30 days.2 This striking increase in susceptibility to the oneogenic effects of polyoma virus is known not to be due to differences in virus growth patterns or differences in antibody formation or concentration.2 That the insensitivity to oneogenesis is related to defense mechanisms operating at the level of the organism is shown by the ability of polyoma virus to transform cells of organ explants to neoplastic growths. 10 It is quite clear that polyoma-induced tumor cells, free of intact virus, are antigenic.'1' 12 They contain a virus-specific "new" cellular antigen, the transplantation-resistance antigen, TR, which is found to be effective in "sensitizing" immunologically competent mice to reject transplants of polyoma tumors and, more significantly, to prevent the induction of neoplasms by polyoma virus in the autochthonous host. 2 Immune mechanisms in both situations apparenltly are of the (cellular, homograft-rejection. type.'3-15 It has been shown recently that the excess

Prevention of virus-induced neoplasms in mice through passive transfer of immunity by sensitized syngeneic lymphoid cells.