Acute infectious gastroenteritis is one of the most important diseases in China. Based on a nationwide surveillance of 21 provinces in 1988, there were 863 million diarrheal episodes per year in China, of which 290 million occurred in children under 5 years of age. Limited studies on human calicivirus (HuCV)-associated gastroenteritis have been performed in developing countries, including China. The hypotheses of this study are that HuCVs play an important role in acute gastroenteritis in children and adults in China and that new HuCV strains with genetic and antigenic properties distinct from currently known strains exist. These hypotheses will be fulfilled by the following specific aims: 1. To determine the role of HuCVs as a cause of severe diarrhea in children and of outbreaks of acute gastroenteritis in the general population by a nationwide surveillance in ten cities and one rural area in China. 2. To determine genetic variation of epidemic strains of HuCVs by cloning and sequencing strains detected in stool specimens from the surveys as well as from historic collections of stool specimens from children. 3. To extend understanding of the genetic variation of HuCVs by cloning the entire capsid gene of newly discovered strains and to develop EIAs by expression of the viral capsid protein in baculovirus.