Two viral agents producing a cytopathic effect (CPE) of degenerative and granular type were isolated from the respiratory tract of pigs with a respiratory disease in two breeding farms in Niigata and Yamagata Prefectures in 1984. They could propagate in porcine kidney (PK) cell culture, as well as in bovine kidney (BK), dog kidney (DK), cat kidney (CK) or Vero (African green-monkey kidney) cell culture with CPE, and agglutinated human type O and porcine erythrocytes. Their replication was not inhibited by 5-iodo-2'-deoxyuridine, indicating that they are RNA viruses. They were not sensitive to ether, chloroform, trypsin or acid (pH3.0). They passed through a 100nm filter with a little difficulty, but did not at all through a 50nm filter. Electron microscopy revealed the presence of spherical particles, 70-80nm in diameter, in the viral specimen. In the cross neutralization test the isolated virus showed the same antigenicity as the Lang strain of reovirus type 1. These properties coincided with those of reovirus type 1. In experimental infection tests, ten-day-old colostrum-deprived piglets showed a rise in body temperature, mild respiratory signs, diarrhea, inappetence and listlessness. Three-month-old conventional pigs showed pyrexia and sneezing for 1-2days after exposure. The serological survey revealed that the positive rate and geometric mean of neutralizing antibody titer against reovirus type 1 were higher than those of reovirus type 2 and 3 at every month of age. Therefore, it seemed that pigs might have been infected mostly with reovirus type 1.