Ten patients with chronic Yersinia enterocolitica infections are described. The initial diagnosis was made by culture, significant agglutinin titres and indirect immunofluorescence (IF) on biopsies. During the chronic phase, culture and agglutinin titres were negative, but specific serum IgA and IgG antibodies reactive with at least two, i.e. the 36 kDa and the 46 kDa, virulence-associated released proteins were demonstrated in nine patients by immunoblot techniques. One patient had only IgG antibodies. The chronically elevated IgA production was the result of chronic stimulation of the gut-associated lymphoid tissue by virulent persistent Yersinia antigen, which was identified by IF with O-specific antiserum and monospecific antiserum to the 46 kDa released protein in biopsies. Virulent Yersinia bacilli were demonstrated in the intestinal mucosa and in the lymphoid tissue of the submucosa associated with macrophages in patients with chronic ileitis and arthritis, in granulomatous centres of lymph nodes in patients with chronic lymphadenopathy and in portal infiltrates in a patient with chronic hepatitis. Recognition of persistent Yersinia infections may have therapeutic implications.