We have studied IL-6 receptor (IL-6R) expression on AML cells from 15 pediatric patients by immunocytochemistry/flow cytometry, reverse-transcription polymerase chain reaction, and Scatchard analysis. High-affinity IL-6R were detected on leukemic cells from 12 (80%) patients. Binding sites per cell ranged from 140 to 3580 (median 920; mean 1240), with dissociation constants of 0.26 to 0.71 nM. We therefore assessed the in vitro sensitivity of IL-6R ؉ AML cells to treatment with a recombinant IL6-Pseudomonas exotoxin fusion protein (IL6-PE 4E ), using the XTT cytotoxicity assay. Leukemic cells from eight patients had ID 50 values (concentration of IL6-PE 4E producing a 50% decrease in cell viability) of Ͻ1000 ng/ml (median, 87 ng/ml; mean, 262 ng/ml). Sensitivity to IL6-PE 4E correlated significantly with receptor number. Normal bone marrow mononuclear cells had undetectable IL6-R expression (Ͻ20 receptors/cell) and were relatively resistant to IL6-PE 4E . To test the efficacy of IL6-PE 4E for ex vivo purging in an autologous stem cell transplantation setting, we incubated primary IL-6R ؉ AML cells with 10 3 ng/ml IL6-PE 4E for 24 h, followed by inoculation into SCID mice. Mice receiving treated cells showed no leukemic engraftment, while all mice receiving untreated or control-treated cells developed leukemia with a median presymptomatic interval of 55 days. In recipients of IL6-PE 4E treated cells, no evidence of occult leukemia was detected by PCR analysis of blood and bone marrow cells at 185 days postinoculation. These data suggest that IL-6R are expressed on leukemic cells from a substantial percentage of pediatric AML patients. Furthermore, leukemic cells expressing high numbers of IL6-R may be sensitive to IL6-PE 4E in an ex vivo purging protocol.