Interferon (IFN)-alpha and -beta are produced by virus-infected cells; IFN-gamma is produced as a primary response of T lymphocytes to mitogenic stimulation, IFN-gamma gene activation being brought about by changes in Ca2+ and phosphatidyl inositol metabolism. IFNs act by binding to cell surface receptors and triggering activation of IFN-responsive genes, probably via specific base sequences located in the 5' non-coding region of such genes, resulting in changes in cell function. Important genes activated in this way are the MHC antigen genes; class I induced by all IFNs, class II by IFN-gamma only. This MHC activation may have important consequences for lymphocyte function.