Breda virus serotype 1 (Iowa) was isolated from a severe outbreak of neonatal calf diarrhoea in which 15% of the diarrhoeic animals died. Two further isolates, one from Iowa and one from Ohio, are antigenically related to the first isolate but belong to the Breda virus serotype 2 group by haemagglutination inhibition and by immuno-electron microscopy (IEM). The majority (88.5%) of cattle are serologically positive for Breda virus by enzyme-linked immunosorbent assay (ELISA). The diagnostic methods of choice should include a combination of electron microscopy, IEM, haemagglutination inhibition or ELISA, in order to identify all serotypes of Breda virus. Serotypes 1 and 2 infect and replicate in crypt and epithelial cells of the small intestine, from the mid jejunum region posteriorly through to the descending colon. Virus replicates in and is released from cells before the appearance of microscopic lesions and clinical signs. Diarrhoea starts within 24-72 hours of infection and death can occur within 36-48 hours after the onset of diarrhoea. The morphology of Breda virus, determined by negative staining and from ultrastructural studies of infected cells, is unique and probably diagnostic. Little is known about the epidemiology, other than that transmission is faecal-orally and may require close contact. Large quantities of virus are released in the faeces.