The present invention provides a method for in-vitro culturing and expanding natural killer (NK) cells in a cell culture medium comprising a population of NK cells, the method comprising a) adding an effective concentration of interleukin-21 (IL-21) at the beginning of the culturing process to said medium, b) adding repeatedly an effective concentration of interleukin-2 (IL-2) and/or interleukin-15 (IL-15) to said medium, and c) adding repeatedly feeder cells or membrane particles thereof to said medium, wherein said feeder cells are B cell derived which are EBV immortalized; and wherein said expansion of NK cells in said cell culture medium is maintained for at least 3 weeks.