Rapid progress has been made over the past two years in the characterization of the biological activities of interleukin-10. Interleukin-10, produced by T cells, B cells, macrophages/monocytes and keratinocytes, alters profoundly the morphology, the expression of MHC class II antigens and the production of cytokines by monocytes which in turn can affect a variety of immunological responses including antigen specific proliferation and cytokine production of both soluble and allo-antigens by T cells, cytokine production by natural killer cells and immunoglobulin production by B cells. IL-10 also directly affects the function and growth of T cells, B cells and mast cells. These characteristics indicate that IL-10 has strong anti-inflammatory activities and may act as a general suppressor factor of immune reactions with consequences for transplantation, tolerance, cancer therapy and infectious diseases.