Persistent virus infections have been implicated in a variety of chronic diseases of man. Measles virus causes a persistent infection in man known as subacute sclerosing panencephalitis (SSPE). Several mechanisms have been proposed to explain this persistence. These mechanisms are virological as well as immunologic and include: defective interfering particles, temperature and other viral mutants, defects in cell and humoral mediated immune responses, as well as antigenic modulation. There are several experimental reasons to involve antigenic modulation in the initiation and likely the persistence of virus in this disorder. In this proposal, the molecular role of antigenic modulation as a means for viral persistence will be critically evaluated.