Faeces from 17 children less than 1.6 years old and 15 adults more than 22 years old were collected during an outbreak of gastroenteritis in a day care nursery and screened for the presence of adenovirus and rotavirus by enzyme immunoassay (EIARA) and other viruses by electron microscopy (EM) and polyacrylamide gel electrophoresis (PAGE). Ten samples (58.8%) from children and one (6.7%) from adults were positive for rotavirus and all samples were negative for bacteria and parasites. No other viruses were observed in EM. An enzyme immunoassay test using monoclonal antibodies (MAb-EIA) to determine the subgroup(s) and the serotype(s) of rotavirus was performed and the results showed that all positive samples belong to serotype 1, subgroup II of group A rotaviruses. In PAGE test all samples had the same profile and the 10 and 11 dsRNA segments corresponded to the "long" profile of group A of rotaviruses. These results corroborated the MAb-EIA results and indicate a sole source of infection. The major symptoms observed were: vomiting (60%), fever (70%) and diarrhoea (100%). In previous years (1989 to 1991) we observed only rotavirus serotype 2 in this same day care nursery, but no outbreak was reported.