Mammalian genes and proteins, designated cytokine synthesis inhibitory factors (CSIFs, now known as Interleukin-10 (IL-10)), are provided which are capable of inhibiting the synthesis of cytokines associated with delayed type hypersensitivity responses, and which, together with antagonists, are useful in treating diseases associated with cytokine imbalances, such as leishmaniasis and other parasitic infections, and certain immune disorders including MHC associated autoimmune diseases caused by excessive production of interferon-&ggr;.