The virulence of influenza-virus strains for Swiss mice was compared with the amount of interferon induced by them in the lungs of the mice and with their sensitivity to inhibition by interferon. The virus strains examined, which included a number of temperature sensitive (ts) mutants, were graded for virulence by their ability to cause lung lesions and to kill mice after intranasal infection. The amount of interferon produced was found to be related to virus growth; the more virulent virus strains grew to higher titres and produced the most interferon. All the strains proved to be equally sensitive to interferon, by three different in-vitro techniques. It was concluded that the virulence of influenza viruses for the mouse is determined by factors other than interferon.


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