In 1957, Isaacs and Lindenmann exposed chick embryo cells to inactivated influenza virus and showed that a protein was produced in response to the virus. This protein, which they called interferon, inhibited infection of the cells by a second virus (1). Interferons were subsequently shown to be members of two. classes of proteins with many different biological activities. The production of recombinant interferons derived from genes from many different mammalian species hasmade significant quantities of these proteins available for clinical trials as well as greatly increasing our knowledge of their biology and genetics.