Forty four pigs with typical characteristics for false positive serolpgical reactions (FPSR) were examined for the presence of Yersinia enterocolitica O:9. The positive reactions were observed in rose bengal test (RBT, N = 23 sera), serum agglutination test (SAT, N = 16), complement fixation test (CFT, N = 9), indirect ELISA (i-ELISA, N= 11) in first, and in RBT (N = 14), SAT (N = 8), CFT (N = 7) and i-ELISA (N = 18) in second examination, respectively. In bacteriological examination Y enterocolitica was confirmed in 12 cases. Six of these isolates were identified with PCR as Y enterocolitica 0:9.