Our experimental data imply the following: (1) An RNA-rich fraction isolated from H37Ra, and intact H37Ra, are effective agents for suppression of tumor growth. (2) For suppression of two chemically-induced tumors with both RNA and H37Ra, direct contact with tumor is necessary. This may not be the case with other tumors in different strains of syngeneic mice. (3) The host's immunological system is a necessary component for inhibition of tumors by our test materials since there is no tumor suppression in immunologically impaired mice.