Immunotoxin BW-2 efficiently kills SCLC and neuroblastoma cancer cells. BW-2 kills (A) HuD-positive H69 cells and (B) neuro-2a, whereas no cytotoxicity occurred in the control (C) HuD-negative K562 cells. Log dilutions of BW-2 (0–5 μg/ml) were added to the cell culture wells (5 × 104 cells/well) and incubated at 37°C for 72 hr. Exposure to equal molar concentrations of streptavidin-saporin (Streptavidin-SP) alone or mAb (16A11) alone served as controls. BW-2 efficiently killed the targeted H69 and neuro-2a cancer cells with great potency ( P < 0.005). SAP-streptavidin alone or 16A11 mAb alone induced only mild non-specific killing at higher concentrations. Data represents the mean of three tests in a single representative experiment repeated 3 times.