On normal monocytes, interleukin 4 (IL-4) down-regulates the production of several proteins that are produced in response to bacterial cell wall lipopolysaccharides (LPS), among others IL-6. In addition, IL-4 was shown to down-regulate the expression of the monocytic differentiation marker CD14, recently identified as a receptor for LPS. Since (autocrine) IL-6 is possibly involved in the proliferation and differentiation of acute myeloid leukemia cells with myelomonocytic differentiation (AML-M4/M5), we studied the effects of IL-4 on IL-6 production and CD14 expression by these cells. We found that IL-4 down-regulates both the expression of CD14 (n = 6) and the production of IL-6 (n = 9) in AML cells. Using Northern blot analysis, we found that IL-4 down-regulates the level of steady-state mRNA for both CD14 (two of three cases tested) and IL-6 (three cases tested).