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The WA69 isolant of infectious bursal disease virus (IBDV) induced cytopathic effects and plaque formation in chick embryo fibroblast (CEF) cultures after serial passages in embryonated eggs and then in CEF cultures. The plaque-forming agent was cloned (designated WA69 clone) and identified as IBDV on the basis of serologic response in inoculated birds and its antigenic relationship to other known IBDV isolants. The WA69 clone replicated rapidly in CEF cultures, reaching peak titers at 48 hours postinoculation, and the virus caused only minimal histologic lesions of the bursa when inoculated into 3-week-old chicks from a specific-pathogen-free flock. The growth of IBDV in CEF cultures with plaque formation may provide a simple in vitro system for virological and serological studies of IBDV.

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