The effect of terdecamycin, a new antibiotic, was evaluated on experimentally induced Mycoplasma hyopneumoniae infection in pigs. Tylosin and josamycin were used as the positive control drugs. Five to 7-week-old pigs were inoculated intranasally with homogenate of pig lung lesions containing M. hyopneumoniae. In experiments 1 and 2, drugs were given in feed for 42 days, from 7 days before inoculation to 35 days after inoculation, and necropsy was carried out 35 days after inoculation. In experiment 3, drugs were given in feed for 10 days, from 3 days before inoculation to 7 days after inoculation and necropsy was carried out 28 days after inoculation. Drug efficacy was evaluated on the basis of clinical signs, lung lesions, (including percentage of gross lung lesion area), isolation of M. hyopneumoniae and growth performance data. The total number of days on which coughing was observed and the mean percentage of lung lesion area in pigs given feed containing terdecamycin at 50 ppm or above were lower than those in unmedicated control pigs. Average daily weight gain in terdecamycin-treated pigs was higher than that in unmedicated control pigs. Treatment with terdecamycin showed equal or better efficacy as compared with treatment with tylosin or josamycin.