We have described in this paper a novel human interferon (IFN) with antigenic and cross-species reactivity of alpha-IFN and physicochemical properties of gamma-IFN. This IFN is produced by normal peripheral blood mononuclear cells during an immune response but has also been associated with autoimmune disease (10). The system described here will be useful in elucidating the biological significance and cell of origin of this IFN.