Three series of tests were performed to challenge the hypothesis that interference with the immune response is a facet of the action of the carcinogen 3-methylcholanthrene. The first series showed that the carcinogen, in the dosages ordinarily used for sarcoma production, could interfere with a weak immune reaction. The second series demonstrated that the same carcinogen could facilitate the growth of first-transplant-generation tumors in isologous mice. Evidence was also obtained that the degree of the effect was related to the specific immunizing capacities of the tumors involved, which was in turn apparently related to the latent period of formation of the primary tumors. Since X-ray treatment of the recipients also improves the growth of first-transplant-generation tumors, the activity probably was exerted, at least in part, via interference with the immune mechanism. The last series provided evidence in favor of an indirect mechanism as a facet of carcinogenic action. The production of sarcomas at the sites of carcinogen-containing pellets was much increased by the remote application of further carcinogen. The evidence, while consistent with an indirect mechanism, was not conclusive. These experiments, augmented by further data from the literature, support the theory that the immune mechanism may normally function to inhibit the development of certain neoplasms. The theory cannot be established, however, unless and until more direct and decisive evidence is obtained.