Published information on rotaviruses as pathogens, the source of virus infection and the method of transmission of infection under normal conditions are reviewed. The antigenic differences between rotavirus isolates from children, calves, pigs, foals and mice are discussed. Bovine rotaviruses isolated in the USA and the UK were shown to be closely related antigenically and the US vaccine strain protected calves from challenge with the UK rotavirus. Nineteen normally reared calves, with 20 or more ZnSO4 units of serum delta globulin, were susceptible to rotavirus inoculation at two days of age. They developed diarrhoea, showed body weight loss but recovered. Three calves with less than 10 ZnSO4 units of serum delta globulin developed diarrhoea and died. In a serological survey of 654 adult cows and calves from three herds, between 2 per cent and 37 per cent of individuals in a group had low rotavirus antibody titres and were probably susceptible to rotavirus infection. These were found in all age groups of animals studied, whether or not the group had suffered a recent rotavirus epizootic. It was not possible to predict whether an epizootic would develop on the basis of a serological survey.