A technique for processing and culturing of human LAK cells using an automated closed system and tissue culture bags is described. To circumvent the inhibitory effects of monocytes on LAK cells the peripheral blood mononuclear cells (PBMC) were pretreated with phenylalanine-methylester (PheOMe). PBMC were obtained from healthy donors by leukapheresis of whole blood. After pretreatment with PheOMe and culturing with IL-2 for 96 h, 60% of the cells remained. PheOMe significantly reduced the number of monocytes (Leu-M3+ cells) from 20-12%. The lytic activity (against K562 and Daudi) of non-PheOMe-treated cells reached a plateau at 72-96 h while PheOMe-treated cells reached maximum activity at 96 h. The total lytic activity/tissue culture bag at 96 h of PheOMe-pretreated cells was significantly augmented in comparison to non-PheOMe-pretreated cells. The present technique allows rapid and simple generation of LAK cells without serum in sterile receptacles suitable for therapy. Additionally, the LAK cell efficacy was improved by reducing the inhibitory effects of monocytes.