Concentrations of rotavirus were studied in serially sampled faecal specimens from 55 patients with acute gastroenteritis, utilizing the immune adherence hemagglutination (IAHA) test. Rotavirus antigen was detected in 45 (82%) out of 55 cases. In the faeces which were positive for rotavirus antigen, the concentrations reached their maximum titer shortly after the onset of the illness and diminished gradually until the 9th day.