The rate of elution of the variant virus from chicken RBC is progressively decreased as the temperature of incubation is increased above 25°C. The activity of the parent virus, on the other hand, is increased as the temperature is increased up to 40°C. The major cause of the decreased activity of the variant at temperatures above 2S°C. is an inhibition of the variant enzyme rather than its inactivation.
The activity of the variant enzyme is stimulated in the presence of strontium, calcium, and barium ions. Manganese has only a slight effect, and magnesium has no stimulatory effect on the elution of the variant virus.
Elution of the variant at 37°C. is progressively inhibited at pH values above 7, while the parent virus is still active at pH 8.
In the absence of calcium the variant enzyme, hemagglutinin, and infectivity are more heat labile than those of the parent virus. The addition of calcium increases the heat stability of all three properties of the variant, and in the presence of calcium the infectivity of the variant is as stable as that of the parent virus.