Small round viruses (SRV) were isolated from the faeces of diarrhoeic calves from three farms. All three SRV preparations caused diarrhoea experimentally in gnotobiotic calves. Each preparation contained viral particles of two morphological types, "astrovirus-like" and "calicivirus-like", and from one preparation the two particle types were separated from each other. The calicivirus-like agent ("Newbury agent") was 33 nm in diameter, and caused diarrhoea in gnotobiotic calves with villous atrophy and D-xylose malabsorption. This virus did not infect cell cultures. The astrovirus-like agent did not cause diarrhoea in two gnotobiotic calves; however, it infected cell cultures (primary calf kidney) and the infected cells immunofluoresced with convalescent gnotobiotic-calf antiserum. The astrovirus-like agents in the three preparations were antigenically related. Experiments in calves showed that there was a degree of cross-protection between the three SRV preparations, as judged by the presence or absence of diarrhoea, but that at least three unrelated pathogens were present.