Specific serum IgA-antibodies, mainly produced in the lymphoid tissue of the gastrointestinal tract (GALT) might exhibit some special characteristics deviating from IgM- and IgG-antibodies: they do not agglutinate nor fix complement. They cannot be determined by usual routine antibody techniques; indirect methods must be used. An antiglobulin-assay was used in an extensive investigation over a two-year period on sera from 8,445 patients to determine IgA-antibodies specific to Yersinia enterocolitica, serotype 3. Y. enterocolitica agglutinins were found in 965 patients, 508 of them with significant titres of greater than or equal to 80. 347 of the 2,111 patients with low titres (10 to 40) had a significantly elevated amount of IgA antibodies. The diagnoses of the IgA-positive patients fell into three groups: acute infections, acute reactive complicatory inflammations, mainly arthritis, chronic inflammatory connective tissue diseases. The IgA-antibody pattern was: In early samples from patients with acute enteric infections they might be the only antibodies present following, largely, during the course of the disease the agglutinating IgM and IgG antibodies, except in patients with long-lasting or chronic complications where IgA antibodies were elevated. It is concluded that determination of specific antibodies of IgA class in cases of Y. enterocolitica infections is an important diagnostic test and, moreover, of prognostic value in the evaluation of chronicity.