Miocamycin (MOM) is a derivative of midecamycin and is metabolized into 4 main metabolites of Mb1, Mb2, Mb6 and Mb12. It is also known that LD50 values of Mb6 were 4,150 mg/kg in male mice and 4,000 mg/kg in female mice but LD0 values in male and female rats were estimated more than 5,000 mg/kg. The object of this study was to examine subacute toxicological effects in male and female rats after repeated oral administration of Mb6 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 study with Mb6 in male and female rats after oral administration at dosage levels of 125, 250, 500 and 1,000 mg/kg for 5 weeks.