There is a great deal of experimental work leading to the conclusion that immuno­ suppression of any kind increases susceptibility to clinical or viral carcinogenesis. Human epidemiology had already given some intimation of the abnormally high incidence of neoplasms in persons suffering from immunologic insufficiency. The first statistics on the incidence of cancer in organ transplant recipients tend to support this thesis. Transplantation can favor metastasis in a recipient who already has a malignancy; similarly, it can encourage the development of tumor cells which may be present in an organ transplanted from a cancerous donor; and finally, the frequency of lymphomas and carcinomas is higher in transplant recipients than in the general population. Immunodepression induced to facilitate tolerance clearly falls under suspicion on the basis of the experimental and clinical findings mentioned above. Anti-lymphocyte serum is not the only suspect-tumors have also occurred in patients treated with prednisone and Imuran alone. Now comes Dr. PENN'S monograph with a detailed account of these phenomena, together with an analysis and discussion. Clearly, induced carcinogenesis in man must be taken seriously. Not only must every precaution be taken in the screening of donor and recipient and the choice of immunosuppressive therapy in order to reduce it to the minimum, there is also the question whether the long-term immunosuppressive treatments inflicted upon patients in whom an auto-immune mechanism is thought to be the cause of the disease-treat­ ments whose efficacy is even more questionable than the mechanism-might not themselves be contributing to malignancy.