We investigated the effect of recombinant human interleukin-4 (rhIL-4) on the in vitro growth of human leukemia cells in liquid culture and 3H-thymidine incorporation and found inhibitory effects on the growth of leukemic cells from patients with Ph1-positive acute lymphoblastic leukemia (Ph1 ALL) and three Ph1 ALL cell lines. However, no inhibitory effects were seen in Ph1-positive leukemic cell lines derived from patients with chronic myelogenous leukemia in blast crisis and various types of Ph1-negative leukemia cells, including B-lineage leukemia cells. In a flow cytometry assay of IL-4 receptor (IL-4R), all three Ph1-positive ALL cell lines showed the presence of IL-4R on their cell surfaces, and the IL-4-dependent inhibition on the growth of Ph1-positive ALL cells was abrogated by the addition of either monoclonal or polyclonal antibodies against rhIL-4. Other cytokines, including IL-2, IL-3, granulocyte-macrophage colony-stimulating factor (CSF), granulocyte-CSF, and IL-6, showed no inhibitory effects on the growth of Ph1-ALL cells, but tumor necrosis factor-alpha (TNF-alpha) and interferon (IFN)-alpha, -beta, and -gamma displayed slight inhibitory effects in a high concentration. The growth inhibition induced by rhIL-4 in the Ph1-positive ALL cells was not abrogated by the addition of antibodies against either IFN-gamma or TNF-alpha. Furthermore, these cells showed no significant production of IFN-alpha, -beta, or -gamma or TNF-alpha after exposure to rhIL-4, thus indicating that the growth inhibition of Ph1-positive ALL cells by rhIL-4 is not associated with IL-4-stimulating production of these factors. rhIL-4 caused significant inhibition of the tyrosine kinase activity in these Ph1-positive ALL cells, similar to Herbimycin A, an inhibitor of tyrosine kinase that inhibited the tyrosine kinase activity in these cells. Our finding suggests that the clinical evaluation of rhIL-4 may offer promising therapeutic possibilities for patients with Ph1-positive ALL.