Interleukin 10 (IL-10) indirectly prevents antigen-specific T-cell activation, which is associated with downregulation of the antigen presentation and accessory cell functions of monocytes, macrophages, Langerhans cells and dendritic cells. In addition, IL-10 inhibits T-cell expansion by directly inhibiting IL-2 production by these cells. These properties of IL-10, together with its capacity to downregulate the production of proinflammatory cytokines and chemokines by activated monocytes, polymorphonuclear leucocytes and eosinophils, indicate that IL-10 is a potent immunosuppressant in vitro. IL-10 has similar activities in vivo. It inhibits lipopolysaccharide or staphylococcal enterotoxin B induced lethal shock in mice. In addition, IL-10 deficient mice develop chronic inflammatory bowel disease, which could be reduced, or prevented by IL-10 treatment. IL-10 also prevented the development of colitis in a SCID mouse model. Collectively, these data indicate that IL-10 has great potential therapeutical utility in the treatment of diseases, such as chronic inflammation, autoimmune diseases, transplant rejection, graft-versus-host disease and sepsis.