Samples of faeces and of serum were collected from pigs of various ages on 21 farms. Faecal samples were cultured on trypticase soy agar containing 5 per cent citrated bovine blood and 400 micrograms per ml spectinomycin, incubated at 42 degrees C in Gaspak jars under an atmosphere of 80 per cent hydrogen: 20 per cent carbon dioxide. Antibody titres to Treponema hyodysenteriae were determined by a microtitration agglutination method using merthiolate-inactivated whole cell antigen prepared from a beta-haemolytic isolate. Results indicated that mean titres in pigs from which beta-haemolytic T hyodysenteriae was isolated were significantly higher than in pigs which yielded isolates of weak beta-haemolytic T innocens or in culturally negative pigs (P less than 0.0225). Mean titres of herds where beta-haemolytic T hyodysenteriae was isolated were significantly higher (P less than 0.005) than the mean titres of either of the other two groups. However, mean titres of herds where no isolates were obtained were not significantly different from mean titres of herds where weak beta-haemolytic T innocens was isolated.