We purified the major influenza virus nonstructural protein, designated NS1, from cytoplasmic inclusions that were solubilized and used to raise antisera in rabbits. One of the antisera was found to be specific for NS1 by complement fixation tests and analyses of immune precipitates. Antiserum to NS1 isolated from cells infected with A/WSN/33 virus specifically precipitated NS1 from extracts of cells infected with seven distinct isolates of influenza A virus representing five different antigenic subtypes. These included A/WSN/33, A/PR/8/34, A/FW/5/50, A/USSR/90/77, A/RI/5+/57, A/Victoria/3/75, and A/Swine /1977/31; however, NS1 from cells infected with B/Lee/40 virus was not precipitated. Radioimmunoassays using radioiodinated NS1 protein from A/WSN virus-infected cells and unlabeled cytoplasmic extracts of cells infected with various strains of influenza virus as competitors indicated significant antigenic cross-reactivities for the NS1 proteins of all influenza A viruses tested. The results suggest a gradual antigenic drift over the 45 yr separating the earliest and most recent virus isolates examined. Thus, compared with the virion neuraminidase and hemagglutinin antigens, NS1 appears to be highly conserved in different influenza A virus isolates.

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