Rotavirus is commonly isolated from diarrhoeic calves and pigs. Bacterium-free faecal filtrates containing rotavirus from five different outbreaks of disease in calves all caused diarrhoea and clinical illness in gnotobiotic calves and five different isolates from pigs were inoculated into gnotobiotic pigs with similar results. The author was unsuccessful in finding an avirulent strain although one of the calf isolates was from a non-diarrhoeic calf. The laboratory strain of calf virus retained its virulence after being passaged seven times in gnotobiotic calves, which included sucrose density gradient purification on two occasions. The calf tissue culture-adapted virus retained its virulence. Rotavirus isolates from humans, calves, pigs and foals were infectious to pigs. Although sharing a common antigen the viruses were separable according to host infectivity, virulence and neutralizing antigens. In both calves and pigs the main lesion was loss of the epithelial cell of the small intestine and stunting of villi. Passive protection of the calf and pig was poor. Circulating antibody was not protective and although high levels of clolostral antibody in the gut lumen at the time of infection protected calves clinically, the antibody level secreted in milk declined 10-fold 48 hours after parturition. Frequently other viruses are found together with rotavirus in cases of diarrhoea. Their role is being investigated.