Interleukin-10 has a variety of biological activities. Murine interleukin-10 inhibits cytokine production by Th2 cells in the presence of macrophages, enhances T cell proliferation, sustains the viability of B cells in vitro, induces class II MHC antigen expression on B cells, enhances mast cell proliferation in the presence of IL-3 and/or IL-4, and inhibits cytokine production by macrophages. Human interleukin-10 inhibits cytokine production by human T cells and reduces antigen-specific human T cell proliferation by downregulation of class II MHC antigen expression on monocytes. cDNA clones encoding murine and human interleukin-10 exhibit a strong homology to BCRFI in Epstein-Barr virus. BCRFI conserves only a part of interleukin-10 activities.