Comparison of cytotoxic properties of naïve NK cells and K562-feeder expanded NK cells. Cytotoxic activities of naïve and K562-NK cells were compared using LDH assay. Cancer cell lines and NK cells were added onto the plates and incubated for 4 hours (A) and 6 hours (B) at 37℃. The data from three independent experiments performed in triplicate are expressed as mean±SD.