Abstract
Four isolates of infectious pancreatic necrosis virus showed ringed plaque morphology on chinook salmon embryo cells, while six other isolates showed clear plaque morphology. One isolate chosen from the ringed-plaque class yielded defective interfering (DI) particles after as few as two virus passages starting with plaque-purified material; another isolate from the clear-plaque class failed to yield detectable numbers of DI particles.