A filterable, heat labile agent has been found in salmonid fishes with presumptively diagnosed and histologically confirmed infectious pancreatic necrosis. In presence of penicillin, streptomycin, and mycostatin, agent produced CPE in cultures of tissues from susceptible host fishes, but not in cultures from a non-host fish or mammals. Isolates of agent were passaged 8 and 9 times, and last dilutions significantly exceeded extinction point noted in original inocula. Sixth and 8th TC passage material was infective for susceptible trout and produced typical disease syndrome. Inocula prepared from such victim fish also evoked characteristic CPE in fish tissue culture. Agent has thus far survived four and a half years storage at
-20 degrees C, and two and a half years in 50% glycerol at 4 degrees C. These facts support the proposition that IPN is a virus-caused disease.