Mice injected with 50-150 μg of Concanavalin A, intravenously, showed increased resistance to infection by Listeria monocytogenes. The number of organisms that lodged initially in the spleen was the same in both untreated and treated mice; yet the subsequent bacterial growth in the treated group of mice was reduced by 10 to 100-fold. The increased resistance to Listeria infection was not seen in thymectomized mice. The treated mice did not show an enhanced specific immunity to Listeria as compared to the untreated mice. In contrast, Concanavalin A enhanced the immune response to hemocyanin but only when administered a day before with it. We conclude that the resistance to listeriosis was brought about by nonspecific stimulation of T lymphocytes by Concanavalin A.