BACKGROUND: Several attempts have been made to expand human NK cells from peripheral blood mononuclear cells (PBMCs). This study examined the selective expansion of NK cells using interleukin 2 (IL-2) plus the K562 cell line, the expression of the NK cell receptors, and the cytotoxic activity. METHODS: The PBMCs from seven healthy volunteers were cultured in a medium containing the IL-2 plus the K562 cell line for 14 days. The expression of the activating and inhibitory receptors on the resting NK cells and the 72 hr-expanded NK cells were analyzed. A flow cytometric cytotoxic assay was used to determined the killing activity of the non-expanded NK cells and the 7 day-expanded NK cells against the K562 target cells. RESULTS: The NK cells from PBMCs expanded 4.5-fold after 7 days, and contained 56.5% CD3-CD56+ cells. The IL-2 or IL-2 plus K562 increased the expression levels of CD158b (MFI, mean florescence intensity), CD158e1/e2 (MFI), and NKp44 (MFI), while it decreased the expression levels of NKp30 (%), CD16 (MFI), and 2B4 (MFI). The non-expanded NK cells lysed 9.0% and 27.6% of the K562 target cells in the 1 : 1 and 5 : 1 effector and target ratio, respectively, and the 7-day expanded NK cells lysed 36.9% and 57.2% of the K562 target cells, respectively. CONCLUSION: The selective expansion of CD3-CD56+ NK cells occurred only during 7 days of culture. IL-2 or IL-2 plus the K562 cells altered the expression of various activating and inhibitory receptors of NK cells, and the cytotoxicity of the expanded NK cells was higher than in the non-expanded cells.