The proposed investigation concerns itself with the observation that organ allograft recipients have a greater risk of developing squamous cell carcinoma of the labial mucosa as well as other types of cancer. I hypothecate that the increased incidence of cancer is, at least in part, a result of the generation of suppressor cells induced by the allograft antigens and that these cells nonspecifically reduce the immune response to nascent tumors arising in the allograft recipients. This hypothesis is supported by preliminary experimental evidence presented by myself (23) and others (24). The need for this hypothesis has been necessitated by the numerous investigations which have shed doubt on the efficacy of the classical immunological surveillance system as an explanation for the observed increased incidence of neoplasia in allograft recipients. In order to support by hypothesis, four major questions are asked in this proposal: 1. Do allografted mice have an enhancement of syngeneic tumor growth? 2. Is the anti-tumor immune respone reduced in the allografted recipients? 3. Is the reduction of the anti-tumor immune response due to suppressor cells within the spleens of the allografted mice? 4. Are the suppressor cells T cells or macrophages?