A family outbreak of gastroenteritis caused by group C rotavirus is described. All five members of the family, with children between 8 and 15 years of age, fell ill with diarrhea. The diagnosis was initially based on the detection of rotavirus RNA showing a typical group C profile in gel electrophoresis in stool samples, and it was serologically verified from patient sera using a cell culture adapted porcine group C rotavirus as a source of standard antigen. All collected serum samples from the family contained IgM and/or IgG class antibodies to group C rotavirus measurable by immunofluorescence antibody test (IFAT). Group C rotavirus specific IgM class antibodies were present in the early serum samples in 3/4 patients. A roller tube neutralization test (NT) was established to demonstrate neutralizing antibodies to porcine group C rotavirus in human sera. These methods can be used to detect serologically group C rotavirus infections.