Three types of interferons have been described: interferon-alpha (IFN-alpha), interferon-beta (IFN-beta), and interferon-gamma (IFN-gamma). An extensive variety of biological effects have been observed for the interferon family including antiviral activity; anti-proliferative activity; stimulation of cytotoxic activities in lymphocytes, natural killer cells, and macrophages; and modulation of the major histocompatibility complex. Only one mouse-IFN-beta gene has yet been described. It codes for a 161 amino acid polypeptide (20 kD). Mouse-IFN-beta binds to the type I interferon receptor. It is not recognized by antibodies directed against mouse alpha interferon or mouse gamma interferon. One unit of mouse beta interferon is the amount of beta interferon which protects 50% of the indicator cell population from viral destruction. One unit of Hycult Biotech Mouse Beta Interferon approximates the bioactivity contained in one unit of the mouse beta interferon standard prepared by NIH (Gb02-902-511).