Sjögren's syndrome is a systemic disorder with unknown etiology, displaying many signs of autoimmunity. Although the basic mechanism of this disease is unknown, a defect in somatic mutagenesis of antibody genes has been suggested. Using a shuttle vector plasmid, we here show that the number of vectors with multiple base changes in a marker gene was reduced in B cell lines from two patients with Sjögren's syndrome (8% in both), as compared with values reported for cell lines from normal human donors (16-27%). This finding suggests that a reduction of the rate of somatic mutagenesis may influence the development of symptoms in Sjögren patients.