Adaptation of Influenza Virus to Mice: III. Development of Resistance to β Inhibitor¹

The egg lines of the Seerey Type A prime and the SSM Type B strains of influenza virus are sensitive to the hemagglutinin (HA)-inhibiting and the in ovo neutralizing activity of the β inhibitor in normal ox serum. Adaptation of the Seerey and SSM strains to CF1, CFW, and NIH mice is accompanied by the development of resistance to the HA-inhibiting and neutralizing effect of β inhibitor. The sudden and concomitant nature of the appearance of extensive pulmonary consolidation in 4 days and the development of resistance to HA-inhibition and neutralization by β inhibitor suggests that these changes are interrelated. Virus resistant to β inhibitor has also been isolated by passage of sensitive virus in eggs in the presence of ox serum.