Slowly reacting enzymatic variants of the Lee strain of influenza B virus were isolated by the serial application of a technique which concentrated the fraction of the virus population obtained by slow elution followed by passage of this fraction at high dilution in chicken embryos. The variants were stable upon serial passage in chicken embryos and did not differ significantly from the parent virus in any of the characteristics tested other than those relating to the virus enzyme.
Enzymatically, the variants had a much slower rate of activity on chicken mucoproteins than the parent virus. Their activity was greater at 25°C. than at 37°C. and was much enhanced by calcium ions and depressed by a calcium chelating agent. The enzyme of the variant virus was more labile to heat than that of the parent virus although their hemagglutinins were equally stable. It was estimated that the variants comprised less than 3 per cent of the original population of the Lee strain of influenza B virus particles.