EACHOF THE INTERFERONS, IFN-ot, IFN-ß, and IFN-7, has long been recognized as cytokines capable of inducing a variety of cells to express antiviral activity. However, if one considers nonviral intracellular pathogens and their specific interaction with one particular host defense cell, the macro-phage, the capacity of the IFNs to induce antimicrobial activity and enhance host resistance diverges considerably." 1 To illus-trate the differing roles of the IFNs in mononuclear phagocyte activation and defense against nonviral pathogens, this review briefly surveys the reported effects of IFN-a, IFN-ß, and IFN-7 in vitro and in vivo and in both animals and man. Five selected activities are addressed.