Serological examinations revealed Y. enterocolitica 0:3 specific antibodies in 34% of 80 children affected by a yersiniosis epidemic at titres of 1/40-1/640 and in all sporadic Yersinia enteritis cases. Similarly, agglutinins were identified for Y. enterocolitica 0:3 and 0:9 in 3.4% of 430 appendectomized children hospitalized with painful syndrome in the right iliac fossa. In 10% of 101 patients hospitalized with arthritis and Reiter's syndrome antibodies to Y. enterocolitica 0:3 (8 cases) and Y. pseudotuberculosis (3 cases) were detected at titres of 1/80-1/640. Our results stress the importance of serological examinations in diagnosing human infections with Yersinia.