IgA and IgG coproantibodies to individual simian rotavirus (SA 11) structural polypeptides were detected in healthy infants in nursery homes. The number of immunoblottable peptides differed from individual to individual. Coproantibodies were also detected at the convalescent stage of rotavirus infection in two patients but not during the acute stage. This method is useful for confirming the diagnosis of rotavirus infection serologically without the need for paired sera.