NK cells expressing anti-CD19 signaling receptors become highly cytotoxic against autologous leukemic cells. Peripheral blood NK cells were obtained from patients with B-lineage ALL in clinical remission. After expansion and transduction, cytotoxicity was tested against autologous leukemic lymphoblasts from diagnostic bone marrow samples. 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. The cytotoxicity of NK cells expressing anti-CD19-BB-ζ receptors (□ and solid lines) was markedly higher than that exerted by NK cells transduced with anti-CD19 truncated nonsignaling receptor (▵ and dotted line; patients 6-8) or empty vector (▵ and dotted line; patient 9).