The incubation of chicken embryos inoculated with original vaccinal strain at different temperature 34°C, 37°C and 40°C had a considerable influence on the pathogenicity of virus inoculated embryos:
1- The mortality index for chicken embryos (MICE) of the original vaccinal strain when the embryos were incuabted at 34°C, 37°C and
Joc were 0.072, 0.028 and 0.46 respectively.
2- One passage of thevirus at 34°C kept thevirus pathogenicity at an indices of 0.05, 0.067 and 0.166 for temperatures of 34°C, 37°C and 40°C respectively.
3- One passage of the virus at 37°C resulted in pathogenicity indices of 0.0, 0.029 and 0.5 for 30C, 370c and 400C and 40°C res pectively.
4- One passage of the 'virus at 40°C resulted in a virus which showed MICE of 0.11 and 0.44 when embryos were incubated at 34°C, 37°C and 40°C respectively.