The in vivo induction of T cell-mediated immunity was studied by infecting mice with two genetically closely related mutants from Listeria monocytogenes, differing only with respect to the secretion of an active SH-dependent hemolysin. It is shown that even minute doses of hemolytic bacteria capable of growing in host tissues easily induced the expression of T cell-mediated immunity, as estimated by the level of delayed sensitivity, adoptive protection and long-lasting immunological memory. On the contrary, nonhemolytic bacteria unable to multiply in host tissues totally failed to initiate the expression of T cell-mediated immunity in vivo. This failure was even observed when mice were repeatedly infected by high doses of nonhemolytic bacteria, allowing to maintain a significant amount of viable bacteria for several days in host tissues. These results mean that the presence of viable bacteria at a significant level in the host is not sufficient per se to induce detectable T cell clonal expansion in the in vivo setting, implying that the process of bacterial growth inside macrophages is required to initiate in vivo the expression of T cell-mediated immunity.