A technique is developed to prepare the optimum number of NK cells for cell therapy by amplifying NK cells from collected blood cells in vitro. The present invention provides a method for amplifying NK cells. This method for amplifying NK cells includes a step for preparing a cell population containing NK cells, a step for removing T cells from this cell population containing NK cells, and a step for culturing the remaining cells from which the T cells have been removed in a medium containing 2500 IU/mL to 2813 IU/mL of IL-2. This method for amplifying NK cells sometimes includes a step for removing hematopoietic progenitor cells from the cell population containing NK cells. The present invention provides a pharmaceutical composition for cell therapy containing NK cells prepared by this method for amplifying NK cells. The pharmaceutical composition for cell therapy of the present invention is sometimes used to treat infections and/or cancer.
