The multiple biological activities present in semipurified lymphokine preparations have made it difficult to assign discrete biological functions to each lymphokine. As a result, the large number of identified lymphokine activities may actually reflect the manifestations of a few factors. While this research has also been hampered by the limited quantities of lymphokines available, hybridoma and recombinant DNA technologies have begun to help overcome these limitations.Macrophage activation has been intensively investigated because it is generally agreed that activated macrophages play an essential role in the defense against microorganisms and in the immune response against neoplasia (1). Macrophage activation mediated by macrophage activation factor (MAF) 1 and gamma interferon (IFN-'y) has been characterized by similar morphologic, metabolic, and functional changes (2-6) including stimulation of nonspecific tumoricidal activities (5), induction of Ia antigen expression (7, 8), increased Fc receptor expression (0, 10), production of plasminogen activator (I 1), and production of hydrogen peroxide (12). Several investigators (5,8,12,13) have postulated therefore that IFN-q~ and MAF may be identical. In support, Schreiber et al. (14) have recently demonstrated biosynthetic and biochemical similarities of IFN-'y and MAF produced by a murine T cell hybridoma (24/G1). Both the antiviral and MAF activities of 24/G1 cultured supernatants were neutralized by anti-IFN-% but not anti- . have demonstrated neutralization of MAF activity with polyclonal antMFN-7, definitive results could not be ascertained since these antisera were prepared from partially purified preparations and could contain antibodies that neutralize other lymphokine activities.In the present report, murine IFN-~ produced by recombinant DNA techniques (18) (>00% pure) was tested for MAF activity; special attention was given

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