Miocamycin (MOM) is a derivative of midecamycin, a macrolide antibiotic and is metabolized into 4 main metabolites of Mb1, Mb2, Mb6 and Mb12. In the previous study, LD0 values of Mb1 were estimated more than 5,000 mg/kg in male and female rats as Mb1 did not exhibited any lethal toxicity even at the maximum physically applicable dose of 5,000 mg/kg. The object of this study was to examine subacute toxicity in male and female rats after repeated p.o. administration of Mb1 for 5 weeks at a daily dosage of 125, 250, 500 and 1,000 mg/kg. It is concluded that no manifest toxicity was observed in this subacute toxicity study on Mb1 in rats.