Cytolytic NK cells are efficiently expanded from PBMC. In the presence of K562-IL15-41BBL (A) expanded cells become significantly enriched (P = 0.0307) in NK cells (defined by CD56+CD3- cells) after 14 days of culture. Expanded cells were evaluated for cytolytic activity using 4 hour 51Cr release assays. Ex-vivo expanded cells from PBMC (■ donor 1 and △ donor 2), but not freshly purified non-expanded NK cells (◇), efficiently lysed allogeneic tumor cell lines derived from breast (MCF-7) and prostate (LNCaP) cancers but not allogeneic or autologous PBMC derived from donor 1 (B). The mean percentage cytotoxicity is shown from triplicate wells. Error bars represent the SD. Lytic activity is likely mediated by NK cells in the expanded cell population (○) since separation in individual populations of NK cells (◇) and NKT/T cells (△) resulted in allogeneic cytolytic activity of the expanded cell population and the purified NK cell population. Little lytic activity was observed in the presence of NKT/T cells alone (C). The mean percentage cytotoxicity is shown from triplicate wells from one representative experiment. Error bars represent the SD. Experiment shown represents one of three individual experiments with three different donors.