Cytotoxic lymphocytes, NK cells and CD8(+) T cells play a pivotal role in the host defense. To reveal the biological function of these cells through establishing a comprehensive gene expression profile, serial analysis of gene expression was performed in human peripheral blood NK cells and CD8(+) T cells. In total, 85,848 tags corresponding to >20,000 different transcripts were sequenced. The genes expressed abundantly in these libraries mostly consisted of genes encoding MHC class I and molecules related to protein synthesis. Among gene transcripts which related to cytotoxicity, granulysin, perforin, granzyme B and alpha-defensin 1 were highly expressed in NK cells. Resting CD8(+) T cells did not express the genes related to cytotoxicity, but expressed abundantly the genes encoding chemokines, tumor necrosis factor family. When CD8(+) T cells were sorted into naive, memory and effector subsets based on the expression of CD45RA and CD27, perforin and granzyme B were expressed in the CD45RA(+)CD27(-) effector subset. Alpha-defensin 1, one of the selectively expressed genes in NK cells, induced migration of naive CD8(+)CD45RA(+)CD27(+) T cells, but not memory CD8(+)CD45RA(-)CD27(+) or effector CD8(+)CD45RA(+)CD27(-) T cells. Furthermore, treatment with IL-15, a stimulator of NK cell development, differentiation, survival and cytotoxicity, rapidly enhanced the expression of alpha-defensin 1 in NK cells. The identification of the genes preferentially expressed in NK and CD8(+) T cell subsets may give important insights into the functions of these cells against virus infection and in tumor immunity.