A process for preparation of an attenuated strain of avian encephalomyelitis virus ("AEV") which may be used as a component of a protective vaccine, which comprises the steps of infecting chick embryo fibroblast ("CEF") cells with AEV, culturing the AEV-infected CEF cells until the AEV has become adapted to the CEF cells and then isolating the AEV product from the CEF cells for use in the vaccine. An attenuated AEV strain produced in this way, and tests showing use as a vaccine in chickens are also described.
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