The effect of recombinat human granulocyte-macrophage colony-stimulating growth factor (rHuGM-CSF) treatment on in vitro interferon (IFN) and tumor necrosis factor (TNF) production in peripheral blood cells of 46 patients with acute myelogenous leukemia (AML) was examined. GM-CSF significantly enhanced virus-induced IFN-alpha production in blood cells (containing 68% of blasts) of 28 patients with M4-M5 AML according to the French-American-British (FAB) classification and also phytohemagglutinin (PHA)-induced IFN-gamma production in blood cells (containing 70% of blasts) of 18 patients with AML MO-M3 type. In control blood cells (25 healthy persons) GM-CSF enhanced PHA-induced IFN-gamma but did not influence IFN-alpha production. In the presence of GM-CSF, TNF-alpha titers induced with lipopolysaccharide were also higher in control blood cells but not in cells of patients with M0-M3 or M4-M5 type of AML. The significance of GM-CSF-enhanced IFN-alpha and IFN-gamma production in antimicrobial and anti-leukemic immune reactions which can develop during GM-CSF therapy is discussed.