Patients who develop reactive arthritis after yersinia enteritis are characterised by high and persisting IgA-anti-yersinia antibodies. We have further analysed the humoral immune response to yersinia in this condition. Total concentrations of IgM, IgG, IgA, and secretory IgA in the serum and specific anti-yersinia antibodies belonging to IgA1, IgA2, or secretory IgA were compared in patients with or without reactive arthritis. In the former group the serum concentration of secretory IgA and of yersinia specific antibodies in all categories studied was raised compared with the level in patients without joint symptoms; the difference was most significant for the IgA-anti-yersinia antibodies with secretory piece or with J chain. The findings suggest that the strong antibody response in the patients developing reactive arthritis is due to a chronic stimulation of the intestinal lymphoid tissue.