Cyclic-AMP-mediated events appear to play an important, perhaps critical, role in the activation and regulation of immune responses. The proposed investigation is designed to substantiate and elucidate these functions, and to improve the use of modifiers of endogeneous cAMP levels for purposes of immunological intervention. The test systems are the antibody response of mice to sheep red blood cells and the cellular immune response of mice to tumor grafts. The known immunoenhancing activity of poly A:poly U will be studied in terms of its postulated effect on the cAMP system. In any case, we shall want to determine when, where and how poly A:poly U functions in the course of the immune response. We shall also search for more potent modifiers of cAMP and determine their efficacy in enhancing host resistance to infectious agents and correcting inherent or acquired immunodeficiencies.