Interleukin-10 (IL-10) has potent anti-inflammatory and immunosuppressive properties. The potential therapeutic benefit may be compromised by the down-regulation of the non-specific immune system and an increased risk of infection. We studied the effects of IL-10 on important functions of native and granulocyte-macrophage colony-stimulating factor (GM-CSF) activated neutrophils and monocytes, namely phagocytosis and membrane expression of the beta superset2-integrins and of the intercellular adhesion molecule-1 (ICAM-1). In order to simulate the in vivo situation closely, we used whole blood flowcytometric assays. The effects of IL-10 (0.05, 1, 10, 100 ng/ml) were compared to those of prednisolone (10 superset-8-10 superset-5 Mol/l), an approved immunosuppressive drug which is known to impair phagocyte function. - Incubation with IL-10 for three hours significantly attenuated the ability of neutrophils to phagocytose E.coli, particularly in lower concentrations. On the other hand, high IL-10 concentrations (10, 100 ng/ml) slightly augmented monocyte phagocytosis. Similarly, expression of the beta subset2-integrins and of ICAM-1 on monocytes was markedly enhanced with IL-10 concentrations in the range from 1 to 100 ng/ml and IL-10 showed strong synergistic effects with GM-CSF in the enhancement of monocyte receptor expression. Neutrophil adhesion molecule expression was not affected. Prednisolone suppressed the phagocytosis of both cell types in a dose-dependent fashion but hardly altered the receptor numbers. Our study indicates that IL-10 can behave as a de-activator as well as an activator on the non-specific immune system, depending on the cell type and the concentration.