Electron microscopy od duodenal mucosa from children with acute non-bacterial enteritis has shown virus particles in epithelial cells. Indirect immunofluorescent techniques applied to the same tissue showed virus antigen localized in the cytoplasm of epithelial cells of the villi. Specific IgM antibody was present in sera from infected patients as early as two days after the onset of symptoms. Virus particles from different patients appeared to share a common antigen. The evidence presented supports our belief that this new virus was the cause of acute enteritis in the children studied.