We have shown previously that the age-associated decline in the ability of mouse spleen cells to generate TNP-specific cytotoxic T lymphocytes (CTL) in vitro results from a loss of Interleukin 2 (IL-2) secreting Lyt 2- helper cell activity, in the face of well maintained Lyt 2+ CTL precursor (pre-CTL) function. In this proposal we describe four approaches to clarifying the nature and mechanism of this age-related immune dysfunction. Initial work will extend our work on IL 2 secretion to a variety of mouse strains, to antigens other than TNP (including alloantigens), and to in vivo immunization protocols. Using complement-mediated lysis as well as a newly developed positive-selection technique, we will learn which cell surface antigens characterize the IL 2 secreting cells, and the other T cells which regulate IL 2 secretion. There are as yet few antigens known to discriminate among the different T cells within the relevant Lyt 2- subclass; we, therefore, describe a new strategy for producing and characterizing monoclonal antibodies useful for this purpose. A third approach will employ a new method for the limiting dilution analysis of IL 2 secreting cells to learn if aging brings about a decrease in the number of antigen-responsive helper cells, or rather a decline in the functional activity of each individual stimulated cell. Lastly, we describe experiments to explore the role played by self-renewal of peripheral, post-thymic T stem cells in maintaining adequate helper cell function throughout the lifespan.