The above sections have provided numerous facts, many of which are conflicting, regarding the changes that occur with increasing age in T lymphocytes. Although it is impossible to state with absolute certainty the alterations that are responsible for decreased proliferation of lymphocytes from elderly subjects, the following summarizes the current status of the data: 1. The interaction of T lymphocytes with foreign stimuli appears to be generally intact. 2. Changes in numbers of CD3+, CD4+, or CD8+ cells before interaction with foreign stimuli or in the density of these markers or of mitogen receptors on the surface of aged T cells have not been consistently observed. When reported to occur, the changes are not sufficient to account for the significant decrease in T-cell proliferation that occurs with increasing age. 3. A defect in the ability of the membrane interaction with foreign stimulus to signal subsequent internal events may occur, because stimulation with phorbol esters and calcium ionophore can result in increased proliferation in some elderly subjects. 4. Decreased accumulation of cytosolic calcium after stimulation of elderly T cells occurs in mice and may be a major component of the defective activation system. This defect appears to be most apparent in the "memory" T cells (T cells expressing high levels of Pgp-1), which increase in number with increasing age. Decreases in Ca++ accumulation have not been observed in humans, but this may be due to different stimuli used. Further, investigation of an increase in "memory" T cells and of their inability to mobilize Ca++ has not been done in humans and rats. 5. Decreases in mRNA for c-myc, IL-2 receptor, and IL-2 have been reported in some, but not all, species. Whether these decreases are the result of decreases in Ca++ mobilization or are independent events in unknown. 6. Decreases in membrane expression of the activation marker RL388 and of TfR have been reported. 7. Lymphokines: a. Decreases in IL-2 production occur in mice and humans, but not in rats. In individuals with decreased IL-2 production, addition of exogenous IL-2 totally restores proliferative ability in only some individuals. Changes in IL-2R expression (number or affinity) may be an additional defect. b. Decreases in IFN-gamma occur in humans, but not in mice or rats. c. No change in IL-1 occurs in any species. Genotypic effects must be considered when evaluating the preceding observations. The heterogeneity among individuals, even within an inbred strain, cannot be discounted.(ABSTRACT TRUNCATED AT 400 WORDS)