In order to determine which components of the salivary duct cells are recognized by the antisalivary duct antibodies, sera of patients with Sjögren's syndrome were screened for immunoreactivity to keratin by using an ELISA test and a Western blot analysis. The autoantibodies which reacted with keratin were demonstrated in 8 of 9 definite cases of Sjögren's syndrome and in 3 of 6 probable cases, and they reacted with different keratin peptides in each case. The results suggest that the antisalivary duct antibodies include antibodies against keratin, which exists ubiquitously as a cytoskeleton in the salivary gland epithelial cells. Detection of antikeratin antibody might be a useful aid for diagnosis of Sjögren's syndro