Due to its similar biological activities to interleukin 10 (IL-10), Epstein-Barr virus (EBV) BCRF1 gene product (viral IL-10: vIL-10) has recently been recognized as an analogue of authentic IL-10. Preincubation of human monocytes with vIL-10, like human IL-10, induced smaller amounts of interferon-gamma (IFN-gamma) mRNA in activated human peripheral blood mononuclear cells (PBMNCs) than nonpreincubation, indicating that vIL-10 acts principally on monocytes. Since the activation of monocytes and their generation of oxidative products are regulated by various cytokines, we examined the effects of vIL-10 on superoxide anion (O2-) production by human PBMNCs and monocytes. Not only PBMNCs but also monocytes preincubated with vIL-10 showed a smaller production of O2-. Inhibition was achieved in a dose-dependent fashion and increased gradually after incubation with vIL-10. Additions of IFN-gamma, macrophage colony-stimulating factor (M-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF), which prime monocyte activation and induce O2- production, were also affected by the reciprocal effect of vIL-10. Thus, vIL-10 production by EBV-infected cells may be involved in the development of EBV-related disorders.