To investigate the in vivo roles of interleukin 15 (IL-15) in host defense against bacterial infection, we constructed IL-15 transgenic (Tg) mice using cDNA encoding secretory or nonsecretiry isoform of IL-15 precursor protein under the control of a MHC class I promoter. The nonsecretory IL-15 Tg mice showed susceptible to avirulent Salmonella choleraesuis infection accompanid by suppression of endogenous IL-15 production. On the other hand, the secretory IL-15 Tg mice exhibited resistant against an intraperitoneal infection with Salmonella choleraesuis and Listeria monocytogenes. NK cells and memory type CD8+Tcells increased remarlably in the IL-15 Tg mice at the early stage of the bacyerial infections accompanied by the higher level of IL-15 production. Furthermore, the bacterial antigen-specific memory CD8T cells were significantly augmented in the IL-15 Tg mice well after the battle against the pathogens has been won. Overexpression of IL-15 in vivo shed light on novel roles of IL-15 not only in early protection but also in maintenance of memory CD8T cells after bacterial infection, which implies IL-15 as an immune adjuvant given with vaccination to enhance its biologic efficacy.