Interferon reduced the production of virus neuraminidase during single-cycle growth of A/HK/1/68 or recombinant X7(F) influenza viruses in chicken embryo cell cultures. Neuraminidase activity, measurable within 6 h after infection, was reduced as much as 70 to 80% below control levels in interferon-treated cultures. X7(F) neuraminidase yield was at least as sensitive a measure of interferon inhibition as vesicular stomatitis virus in the standard plaque-reduction assay. Interferon titre, expressed as the 30% neuraminidase reduction dose or NRD, is derived from semi-logarithmic sigmoidal dose—response curves. The advantages that recommend the neuraminidase reduction bioassay for interferon assay include the precision of the enzyme assay (±1.0%), the precision of measurement of replicate samples of interferon (±9%), the reproducibility of interferon titres obtained sequentially (±20%), as well as the rapidity and economy of the method.


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