Experimental murine listeriosis was used as a model to investigate the immunological basis for the age-associated decline in antimicrobial immunity. The reduced capacity of protective T cells from Listeria-immune senescent mice to adoptively immunize normal syngeneic recipients could not be attributed to inhibition of their activity by suppressor cells. Radiolabeled enriched splenic T cells from Listeria-immune young or senescent donors exhibited an identical distribution pattern after an intravenous infusion into young recipients. Moreover, cells from Listeria-immune young donors showed markedly greater protective capacity than cells from senescent immune donors whether the cells were transferred to young or senescent recipients. Dose-response analysis of protective T cells revealed that in response to immunizing infection (a) senescent mice generated 10-fold fewer protective T cells, and (b) protective T cells from senescent mice were 100-fold less efficient than cells from young mice.