Twenty-five-day-old broiler chickens were inoculated intramuscularly or orally with a suspension prepared from the livers of chickens naturally affected with hydropericardium syndrome (HPS), and uninoculated chickens were kept in the same room with the inoculated birds. The mortality rates in the chickens inoculated intramuscularly or orally were 100 per cent and 30 per cent, respectively. The corresponding groups of uninoculated chickens had mortality rates of 60 per cent and 53.3 per cent, respectively. Hydropericardium was a consistent and prominent lesion in the dead birds, and there were gross lesions in the liver and kidney. Microscopical lesions were present in the liver, kidney, bursa of Fabricius and spleen. Some hepatocytes contained basophilic, intranuclear inclusion bodies.