Although IFN proteins were recognized first for their potent antiviral properties, it has now been established that they may profoundly affect other vital cellular functions. The IFNs are divided into three main classes, alpha, beta, and gamma, and are defined by their differences in amino acid sequences, physicochemical properties, and induction by different agents from different cell types. The inducing agents include viruses, bacteria, bacterial products, polymers, low molecular weight compounds, and antigens or mitogens. Studies on the mechanisms of action of IFNs have mainly been focused on their antiviral actions. However, many of the facts revealed by these studies are equally relevant for understanding other actions of IFN. IFNs are extremely potent, they interact with specific receptors, and they induce the expression of specific genes, the products of which mediate their various actions. There is almost a complete lack of knowledge of what happens between the interaction of IFN with its receptor and induction of new RNA synthesis. However, we are beginning to understand how some of the IFN-inducible enzymes impair viral replication. The discovery of the dsRNA-dependent enzymes has implications beyond the IFN system. It is quite possible that they are used for other physiologic regulatory systems as well. The identities and functions of many other IFN-inducible proteins remain to be elucidated. Principally, IFNs alpha and beta are cytokines in that they may be produced by the cellular components of the immune system and have immunoregulatory effects on the cells of the immune system. These effects include enhancement of surface structures such as histocompatibility antigens, pleiotropic hormone-like effects, and stimulation or inhibition of the activities of a number of different effector cells such as B cells, T cells, macrophages, and natural killing cells. IFN levels may be below detection and yet mediate important biologic functions. Perhaps the most interesting IFN subtype regarding immunoregulation is IFN gamma, which is a product of T lymphocytes. Few drugs have stimulated as much research interest or clinical promise as the IFNs. Clinical trials in patients have shown most promise in coryza, herpes virus infections, papilloma virus tumors, hairy cell leukemia, multiple myeloma, and renal cell carcinoma. IFN gamma employed alone and in combination with IFN alpha may dramatically increase IFN's activity. IFN treatment combined with chemotherapy also may give enhanced antitumor activity.(ABSTRACT TRUNCATED AT 400 WORDS)