An anchorage-dependent Wilms tumor cell line HFWT was found to stimulate selective and remarkable expansion of human natural killer (NK) cells from human peripheral blood mononuclear cells (PBMC). After PBMC of healthy donors were cultured on irradiated HFWT cells for 10 - 21 days, the lymphocytes expanded 58- to 401-fold. This NK cell expansion required direct contact of PBMC with live, but not fixed, HFWT cells. The PBMC from an end-stage brain tumor patient also expanded 156-fold, whereas those cultured with irradiated NK-sensitive K562 grew only 30.5-fold. CD16+ CD56+ NK cells accounted for more than 70% of the population expanded on HFWT cells. No essential difference in expression of NK receptors was observed in the expanded NK cells on HFWT and K562 and without feeder cells. The expanded NK cells killed not only fresh HFWT cells but, unexpectedly, also MHC class I-expressing autologous brain tumor cells at an effector/target ratio of 4 for 24 h. These results will contribute to the development of a large-scale preparation method for human NK cells, which will aid studies of NK cell biology and possible treatment of brain tumors.