The efficacy of in-feed medication with tylosin for the treatment of enzootic pneumonia was examined in an experimental Mycoplasma hyopneumoniae infection model. One group of 10 conventional M. hyopneumoniae-free pigs was inoculated intratracheally with a highly virulent field isolate of M. hyopneumoniae; a second group of 10 pigs was inoculated in the same way and after 12 days was given tylosin at 100 mg/kg feed for 21 days; a third group of 10 pigs was inoculated with sterile culture medium, and these pigs were not given tylosin. The pigs were examined daily for clinical signs and each pig was given a respiratory disease score. Thirty-three days after they had been infected the pigs were euthanased, the lung lesions were quantified and samples of lung were processed for immunofluorescence testing for M. hyopneumoniae. The mean (sd) respiratory disease and lung lesion scores were significantly higher (P<0.05) in both the infected groups than in the uninfected group. Between 23 and 33 days after infection the mean respiratory disease score of the pigs treated with tylosin was 0.54 (0.22), significantly (P<0.05) lower than that of the infected pigs which were left untreated, 1.54 (0.46); similarly, their average lung lesion score, 1.72 (1.20), was significantly lower than that of the untreated pigs, 5.27 (3.85).