Nieschulz1 and Alexander2 independently showed that horsesickness virus multiplies in the brains of Swiss albino mice. Alexander3 reported later the successful cultivation of mouse-adapted horsesiekness virus in the brains of guinea pigs, rats and other rodents. He also succeeded in cultivating a mouse-adapted strain of virus in embryonated hen's eggs, in which multiplication of the virus occurred in the brain of the embryo without producing any specific mortality4. McIntosh5 propagated viscero-tropic horsesickness virus strains in 8-day-old embryonated hen's eggs at an optimum temperature of 32° C. Embryo mortality occurred regularly during the initial passages.