A considerable variabilility in the effects of sample handling on NK cytotoxicity has been observed. Using flow cytometry, NK cytotoxicity assays and lymphocyte subset analysis of Ficoll-Hypaqueseparated peripheral blood mononuclear cells (PBMCs) isolated from whole blood stored under various conditions were performed. The NK cytotoxicity of samples in heparin tubes stored overnight at 4 and 22°C, as well as at 22°C in acid citrate dextrose (ACD) tubes, was lower than that of a fresh sample. However, the NK cytotoxicity of samples in an ACD tube stored at 4°C was similar to that of a fresh sample. Based on lymphocyte subset analysis, samples in an ACD tube stored at 4°C showed a lower percentage of CD3+ T cells and a higher percentage of CD16/56+ NK cells compared to samples stored under other conditions. The NK cytotoxicity of fresh samples and samples in ACD tubes stored in a Styrofoam cooler box did not differ significantly; however, the differences were inconsistent. Overnight storage of peripheral blood in ACD tubes at 4°C is optimum for retention of NK cytotoxicity, the level of which is similar to that of fresh blood. This may be associated with an increased NK-cell fraction in Ficoll-Hypaque-separated PBMCs after overnight storage.