The last ten years have seen an explosive growth in our understanding of IFN gamma. The cloning of the cDNAs for IFN gamma and its receptor have made available large amounts of highly purified recombinant IFN gamma and soluble IFN gamma receptor. In addition, highly specific neutralizing monoclonal antibodies have been generated to both of these proteins. Using these reagents, IFN gamma and the IFN gamma receptor have been characterized on a molecular basis. Structure-function studies carried out on these proteins have identified key molecular regions that are required for biologic activity. Moreover, a great deal is now known concerning the physiologic role that IFN gamma plays in vivo. In this review we focus on the new developments in the areas of IFN gamma biochemistry and biology and pay particular attention to the IFN gamma receptor and three aspects of IFN gamma biology that are of special interest to immunologists: host defense, inflammation, and autoimmunity.