Six-month oral chronic toxicity study and two-month recovery tests on rokitamycin (TMS-19-Q) were performed in Beagle dogs at daily doses of 90, 180 and 360 mg/kg. The results were summarized as follows: No death occurred to any animals in these experiments. In higher dosage groups (180 and 360 mg/kg), vomiting, salivation and reduction of body weight gain with decrease in food intake were observed. Body temperature and pulse rate were normal, and ECG, sperm and vaginal smear tests, urinalysis, blood and biochemical examinations also revealed normal values. No marked changes in organ weights, necropsy, histopathological and electron microscopic observations were detected.