Interferons were discovered from their antiviral effects. It is now known that they are the products of families of genes found in many vertebrate species and that they bind to cell surfaces and produce a range of effects, varying from an antiviral state to increased expression of tissue transplantation antigens, decreased cell division, and modified immune responses, among others. Interferons are induced by exposure to viruses and double-stranded RNA, and one class, gamma interferon, is a lymphokine produced after a mitogenic or antigenic stimulus. The genes for human alpha, beta, and gamma interferons have been cloned and sequenced, and some have been expressed. When given parenterally to humans, interferons cause signs and symptoms such as fever, tachycardia, and malaise, and they may well be responsible for symptoms of this sort in viral infections. Preparations of natural and recombinant alpha interferon have been shown to be active in prophylaxis and treatment of certain viral infections.