A it has been shown that killer cell immu- noglobulin-like receptors (kirs) on peripheral lym- phocytes are upregulated by interleukin-2 (il-2), which activates natural killer (nk) activity, it has not been demonstrated whether the expression of kirs is related to nk activity. therefore, we investi- gated the association between the kir expression on lymphocytes and nk activity. cd158a/b expression on lymphocytes obtained from 37 subjects was analyzed using flow cytometry. simultaneously, nk activity was measured each sample using a cr- release assay. additionally, lymphocytes were cul- tured in rpmi 1640 medium with or without il-2 for 48 h, and then their cd158a/b expression and nk activity was analyzed. cd158a/b expression was significantly correlated with nk activity. especially, the percentage of cd16 � cd158a and cd8 cd158a/b cells in lymphocytes showed a highly significant correlation with nk activity. however, analysis of cd8 and cd16 cells revealed that there was only a significant correlation between the percentage of cd8 cells among only cells and nk activity. the upregulation of /b cells in response to il-2 tended to be related to the increase of nk activity, but the relationship was not significant. in conclusion, the level of kir expression was correlated with nk activity, and il-2 treatment resulted in an increase of nk activity as well as kir expression, suggesting that upregulation of kirs enhances the ability to sort target cells, such as virus-infected cells from unin- fected cells, according to major histocompatibility complex class i expression. key words: killer cell immunoglobulin-like receptors, natural killing activity, major histocompatibility complex class i, inactivation mediators of inflammation, 12 (2), 117 121 (april 2003)