Since interleukin-4 (IL-4) specifically inhibits monocytic colony formation in human bone marrow cultures, we investigated whether a similar inhibition could be observed in cultures of optimally stimulated acute myeloid leukemia cells with myelomonocytic differentiation (AML-M4/M5). Sensitivity to IL-4 was tested in 19 cases of AML-M4/M5, using both a 3H-thymidine incorporation assay and a clonogenic assay. Proliferation was stimulated with a combination of IL-3, granulocyte-monocyte colony-stimulating factor (GM-CSF), and conditioned medium from phytohemagglutinin-stimulated leukocytes (PHA-CM). In 13 out of 14 evaluable cases, IL-4 inhibited 3H-thymidine incorporation; stimulation was seen in one case. Using a clonogenic assay, IL-4 inhibited colony formation in all evaluable cases (n = 7). Addition of IL-6 did not alter the observed inhibition by IL-4 in 9 out of 10 cases tested. We conclude that IL-4 inhibits the proliferation of optimally stimulated AML-M4/M5 cells in most cases tested, and that this effect is not generally caused by inhibition of autocrine IL-6 production.