An immunoglobulin G monoclonal antibody (mAb C9E8) recognising a genus-specific epitope on the 26 kDa protein of porcine Serpulina species organisms was used in a simple colony blot assay to detect Serpulina in cultures grown directly on blood agar plates from pig faeces and tissues. The mAb detected even a few colonies of the organism in the presence of an abundant growth of non-Serpulina organisms. The whole procedure was completed in less than three hours. A total of 123 strains of S hyodysenteriae and S innocens were correctly identified by the colony blot assay whereas all the 26 non- Serpulina Gram-negative organisms commonly isolated from faecal material or tissues of pigs remained negative. The assay was rapid, highly specific and sufficiently reliable to be used with confidence for identifying porcine Serpulina colonies directly on blood agar plates.