Monocytes and macrophages play a central role in viral infections, as a target for viruses and in activation of both innate and adaptive immune responses. Epstein–Barr virus (EBV) has evolved elaborate strategies to dampen the immune system and to persist within the host. There is evidence that the product of the BCRF-1 open reading frame of EBV, viral interleukin-10 (vIL-10), inhibits the capacity of monocytes/macrophages to induce T cell activation, but the full mechanism of this effect is unknown. To determine whether this effect might involve modulation of the expression of accessory molecules known to be important in T cell activation, we analyzed by flow cytometry the influence of vIL-10 on the basal as well as on IFN-γ-induced up-regulation of HLA molecules, ICAM-1, and two members of the B7 family B7.1 (CD80) and B7.2 (CD86) at the surface of human monocytes. Viral IL-10 in a concentration-dependent manner inhibited both basal- and IFN-γ-induced HLA-class II, ICAM-1 (basal levels of ICAM-2 and ICAM-3 is unaffected), CD80, and CD86 up-regulation when added simultaneously with IFN-γ. In contrast, complete inhibition of HLA-class I expression on monocytes/macrophages occurred only when vIL-10 was present 2 h prior to the addition of IFN-γ, implying that vIL-10 affects an early step in the IFN-γ signaling pathway. As both monocytes and macrophages can be infected by EBV, we propose that vIL-10-mediated impairment of monocyte/macrophage antigen-presenting function could help the virus-infected cells to avoid detection by the host's T cells on one hand and contribute to its immunosuppressive properties on the other.