IgA antibodies to Yersinia enterocolitica were demonstrated in the sera of 13 patients with severe versinia arthritis who were studied six to eight months after an acute infection with Yersinia. Four of the patients were monitored for two to three years, and they continued to demonstrate these antibodies. Only one of 12 control patients (individuals with yersinia infection without arthritis) had IgA antibodies specific to Yersinia six to eight months after the acute infection. The persistence of IgG antibodies was also in direct correlation to the occurrence of arthritis, but not as clearly as was the persistence of IgA antibodies. Antibodies of the IgM class persisted in most cases for only one to three months and always disappeared during the first six months after the onset of the infection. Thus, the demonstration of IgA antibodies to Yersinia is important in the diagnosis of yersinia arthritis, and the occurrence of IgM antibodies indicates a recently acquired infection with Yersinia.