The mammalian immune system appears to play an essential role in preventing the development, and/or in controlling growth and spread, of malignant tumor cells. Antigenic properties of neoplastic cells and defense mechanism of the tumor host are briefly reviewed. The relative importance of immune reactions and other "non-specific" means of host defense with regard to tumor development and growth are discussed in the light of available experimental and clinical evidence. An attempt is made to incorporate these considerations into a hypothesis relating to the development of intestinal neoplasms.