RECENTLY it has been shown1 that a virus isolated from cattle with signs of ‘mucosal disease’2 is antigenically related to the virus of bovine virus diarrhœa3. In northern Sweden, a type of mucosal disease (‘Umea disease’) was found to be accompanied or preceded by a respiratory disease. In paired serum samples from diseased animals increasing titres of antibodies were demonstrated against both bovine virus diarrhœa virus and para-influenza 3 virus4, but only para-influenza 3 virus was isolated5. The incidence of infections with bovine virus diarrhœa virus among Swedish cattle seems to be about 20 per cent as judged from the results of a serological survey in 1959–604. In the present work the properties on which Cooper6 bases his classification of animal viruses have been investigated for bovine virus diarrhœa virus.