IgG subclass-specific antibody responses to a human subgroup 2 rotavirus were studied in 26 children by ELISA by use of monoclonal antibodies specific to the four human IgG subclasses. One hundred twenty-nine serum samples were obtained before, during, and after an episode of rotavirus-induced diarrhoea in these patients. When these sera were investigated, an increase in IgG1 and IgG3 subclass-specific antibodies was detected in all 26 patients. IgG3 antibodies reached a peak concentration 1 week after rotavirus was detected in faecal samples and then progressively declined over the following months, whereas the peak concentration of IgG1 subclass antibodies was found 2 months later and seemed to persist thereafter. IgG2 rotavirus-specific subclass antibodies were never found and IgG4 subclass antibodies were detected only in sera from seven of the 26 patients.