Summary A synthetic anionic polymer, polyvinyl sulfate (PVS) showed in vitro and in vivo antiviral activity. The polymer did not induce interferon in vitro in L929 or chick embryo cells but interferon could be detected in the circulation of mice treated with polyvinyl sulfate. The mechanism of protection of PVS has not been completely elucidated and the possibility that it exerts multiple modes of activity cannot be excluded.