Determination of antibodies against Eubacterium suis (E. suis) in the serum of pigs was carried out by an indirect immunofluorescence test. Therefore 35 sows with urinary tract infections as well as 43 healthy control sows were investigated. E. suis could be detected in the urine of 19 sows. Serological results were compared with the bacteriological examinations. To reach a specificity of 100% for detection of animals actually infected with E. suis, titres from 1:16++ can be accepted as positive. Therefore sensitivity gave a level of 78.9%. Seroconversion and development of antibody titers were studied on 15 sows, which were experimentally infected with E. suis. Antibodies could be shown 3 weeks after infection at the earliest, they were preferably demonstrated in sows with a haemorrhagic cystitis. No correlation existed between the affection of the kidneys and the development and the level of titres respectively. Serological investigations can turn out negative in spite of E. suis infection, if immunological response still has not taken place or development of antibodies has failed to appear after short-time infection of the bladder.