Natural killer (NK) cells have been demonstrated to be activated by interferon (IFN). However the mechanisms of this activation are still not well known. The aim of this study is to investigate the mechanisms of NKstimulation by IFN against human leukemia cell line K562, human renal cell carcinoma cell line CaKi 1, and human prostatic carcinoma cell line DU145, using the methods of 51Cr-release assay and single cell cytotoxicity assay. From the results of single cell cytotoxicity assay, frequency of active NK cells among peripheral blood lymphocytes (PBL) against each cell line increased significantly by preincubation of PBL with 2, 000 IU/ml of human beta interferon (HuIFN-β) for 12 hr. Recycling capacity of NK cells was measured combining single cell cytotoxicity assay and 3 hr 51Cr-release assay, using K562 cells as target cells. By this measurement maximal recycling capacity (MRC) increased from 10.9±2.2 to 17.8±2.9 (n=7, p<0.01). It was concluded that stimulation of PBL with HuIFN-β resulted in increase of active NK cells and recycling capacity, i.e. function of individual NK cells to recycle after killing of the initially encountered tumor cells.