From 10 days before the expected date of farrowing onwards, 97 sows infected by Mycoplasma hyopneumoniae and Treponema hyodysenteriae were given tiamulin daily at a dosage of 20 mg/kg bodyweight via the feed. Three days before farrowing the sows were washed with a disinfectant and transferred to an isolated farrowing house. The sucking piglets remained with their dams for five days, during which time the sows continued to receive the tiamulin-containing feed. The sucking piglets also received tiamulin daily at a dosage of 30 mg/kg bodyweight. At six days old the weaker piglets of the litter were returned to the original herd, together with their dams. A total of 574 piglets of about 1.5 kg bodyweight each were transferred to an isolated and previously disinfected pig farm and reared there. A total of 13.8 per cent of these pigs died by 50 days old. On the isolated farm, 10.9 per cent of the 829 second generation piglets born to the 101 first generation sows, died up to the age of 50 days. On the isolated farm about 2000 pigs were subjected to repeated clinical, pathological and laboratory examinations for M hyopneumoniae, T hyodysenteriae, Aujeszky's disease virus and Leptospira species during the three year period of study. No evidence of infection with any of these agents was found in the 2000 pigs of the isolation herd, although the original sow herd had been latently infected by these pathogens. No maternally derived antibodies against these pathogens were detectable in sera of three-day-old sucking piglets of the second and third generations.