The cytolytic activity of cells expressing natural killer cell receptors (NKRs) depends on the balance between stimulatory and inhibitory signals. We investigated both inhibitory NK receptor (CD94/NKG2A) expression and stimulatory NKR (NKG2D) expression on T cells after stimulation with cytokines (IL-12 or IL-15). Cytolytic NKR-expressing CD8(+) T cells were expanded from normal adult peripheral blood mononuclear cells using anti-CD3 monoclonal antibody and cytokines (IL-12 or IL-15). The proportion and absolute number of CD94/NKG2A-expressing T cells expanded by IL-12 were significantly larger than those of the cells expanded by IL-15. On the other hand, the proportion and absolute number of NKG2D-expressing T cells expanded by IL-15 were significantly larger than those of the cells expanded by IL-12. The proportions of NKG2D and intracellular granzyme A expression in CD94-expressing cells were much more increased in PBMCs cultured with IL-15 than those of cells cultured with IL-12. A real-time polymerase chain reaction assay showed that there was a 1.68-fold increase in NKG2D mRNA expression level and a 1.37-fold increase in DAP10 mRNA expression level in CD94-expressing cells expanded by IL-15 compared with those of the cells expanded by IL-12. The cytolytic activity levels of purified CD94-expressing cells from 8-day culture with IL-15 tested against (51)Cr-labeled K562 cells by standard 4-h (51)Cr release assays without prior sensitization were much higher than those of cells from 8-day culture with IL-12. IL-15 appears to be able to enhance the cytolytic activity of CD94/NKG2A-expressing cells through induction of NKG2D and intracellular granzyme expression much more efficiently than does IL-12.