Norwalk virus (NV) has recently been assigned as the prototype species for a new genus in the Caliciviridae, provisionally named ?Norwalk-like viruses.? The ?Norwalk-like? human caliciviruses are the major cause of nonbacterial epidemic gastroenteritis that occurs in family, school, institutional, or community-wide outbreaks affecting adults and school- aged children. The role of these viruses as agents of gastroenteritis in infants and young children has not been established, although we previously reported evidence for a high infection rate with Norwalk virus in Finnish infants and young children. A major thrust of our work this year was to establish a collaborative effort with investigators at the Children?s Hospital in Washington, D.C. to address the role of human caliciviruses in pediatric diarrheal disease. Serum and stool samples collected as part of a rotavirus vaccine study in Washington, D.C. conducted between November 1989 and December 1991 will be analyzed for evidence of Norwalk-like human calicivirus infection. These studies involved 51 healthy infants and young children who received either rotavirus vaccine or placebo, and consequently were monitored for the development of diarrhea during the post-vaccination and surveillance periods. Serum samples were obtained periodically and stool samples were collected when diarrhea was reported. Our preliminary analysis of these sera with Norwalk, Hawaii, and Toronto human calicivirus immunossays indicates that infection with ?Norwalk-like caliciviruses? occurs frequently in infants and young children in Washington, D.C. This high infection rate may reflect a more important role for these viruses as agents of diarrheal disease in this young age group than previously recognized. A major accomplishment this year was the generation of VLPs from the Snow Mountain virus (SMV) in Genogroup II, that previously had been shown to be a distinct serotype of human calicivirus by IEM. The ORF2 and 3 were cloned together into a baculovirus transfer vector and the expressed SMV capsid protein self- assembled into VLPs. An enzyme-linked immunosorbent assay (ELISA) was developed to detect serologic evidence of infection with this virus. - diarrhea, gastroenteritis, Norwalk virus, caliciviruses, virus-like particles, vaccines, epidemiology