Sera of 1283 patients from hospitals and medical centers of Wrocław were examined for Y. enterocolitica antibodies. In 29% examined sera anti-Y.enterocolitica antibodies of titer 1:40 and greater were noted. The agglutinins against serotype IA(3) were in 82.5%, against V(9) in 8.6% and against both serotypes mentioned above in 8.8%. In the group of sera positive for serotype IA 70.2% of sera gave positive reaction using antigen made from the humans strains and 9.1% using antigen prepared from swine Y.enterocolitica isolate. Patients with the positive titer of agglutinins against Y.enterocolitica in their sera mainly shoved the symptoms of gastroenterocolitis (54.8%) and arthritis (14.5%). The highest percentage reactions was in children aged 7-14 years (47.3%) whereas in adults it was lover (28.8%).