NK cells expressing 4-1BB-augmented chimeric receptors show powerful cytotoxicity against leukemic cells from patients. Expanded primary NK cells expressing chimeric receptors were incubated for 4 hours with leukemic cells from children with different subtypes of B-lineage ALL (patient [Pt] 1, hyperdiploid 47-50; Pt 2 and Pt 5, t(4;11)(q21;q23); Pt 3, t(14;?)(q32;?); Pt 4, der8, t(8;?)) at the indicated E/T ratios. Each data point represents the mean (± SD; n = 4) percentage of ALL cell killing after culture as compared to that of parallel cultures without NK cells. With the exception of the results obtained in patient 2 at a 1:1 ratio, the cytotoxicity of NK cells expressing chimeric receptors containing 4-1BB was significantly higher than that induced by receptors without 4-1BB (P < .005).