Many experimental evidences indicate that infection with Trypanosoma cruzi delays or inhibits the growth of malignant tumors in different strains of mice and in rats. These facts were verified by different workers. Although earlier workers proposed that this effect would be due to a toxin of T. cruzi, most of the accumulated evidences do not agree with such proposal. This present hypothesis agrees with the experimental data and proposes that the liberation of many endocellular antigens by destruction of some cancer cells, infected with T. cruzi, gives rise to an autoimmune response against antigens of analogous cancer cells, which limits or inhibits tumor growth. This point of view is supported by experimental studies on Chagas' disease which showed the role of T. cruzi, to induce autoimmune reactions against target organs of the disease. On the basis of this hypothesis I postulate a new way to stimulate the immune system of the host against cancer.