The cytokine interleukin-10 (IL-10) has several important activities on cells of the immune system. IL-10 profoundly suppresses activation of macrophages, inhibiting their ability to secrete cytokines and serve as accessory cells for stimulation of T cell and natural killer (NK) cell function. IL-10 also plays a role in stimulating proliferation and differentiation of B cells, mast cells, and both mature and immature T cells. At least two herpesviruses harbor analogs of the IL-10 gene; the Epstein-Barr virus (EBV) homolog (BCRF1, viral IL-10, vIL-10) shares several of the cellular cytokine's activities, one or all of which may be important in the host-virus relationship. This article reviews recent studies on the function of IL-10 and discusses the initial characterization of its receptor.