A comparative toxicity study on macrolide antibiotics, rokitamycin (TMS-19-Q) and josamycin, was performed in male rats. These drugs were given orally for 1 month at daily doses of 250, 500, 1,000, and 2,000 mg/kg. No animals died in the study. No abnormal symptoms or changes in body weights and food consumption were observed. Results of urinalysis, hematological analysis and biochemical analysis of serum and organ weights were normal except that a dose-dependent hypertrophy of caecum was observed in all dosage groups. Necropsy revealed no notable drug related abnormalities except in caecum of all test groups. Pathological examination found no significant drug-related abnormalities in the test groups. No toxicological signs were detected in the group administered with 2,000 mg/kg of either drug and no difference was found between the 2 drugs.