<;P>;PROBLEM TO BE SOLVED: To provide an NK activity enhancer, a lymphocyte or monocyte having enhanced NK activity (NK activity enhanced lymphocyte or NK activity enhanced monocyte): a cellular immunity preparation, transfusion and blood preparation containing the NK activity enhanced lymphocyte or monocyte: and methods for preparation of the enhancer, etc. <;P>;SOLUTION: Nano-bubble water containing bubbles having a number-average diameter of &le;300 nm determined by dynamic light-scattering method is used as the NK activity enhancer. The NK activity enhanced lymphocyte or monocyte can be prepared by culturing lymphocyte or monocyte separated from a living body in the presence of the NK activity enhancer. <;P>;COPYRIGHT: (C)2010,JPO&INPIT