Miocamycin (MOM) is a derivative of midecamycin and is metabolized into 4 main metabolites of Mb1, Mb2, Mb6 and Mb12. In the previous studies, we estimated LD50 values of Mb6 in male and female mice after single oral administration. The LD50 values were 4,150 mg/kg in male mice and 4,000 mg/kg in female mice, respectively. In the present studies, we evaluated acute toxicity and estimated LD50 values of Mb6 in male and female rats after single oral administration. Observations were continued for 1 week after treatment. It is concluded that LD0 values of Mb6 in male and female rats, were estimated more than 5,000 mg/kg as Mb6 did not exhibit any manifest acute toxicity even at the maximum physically applicable dose of 5,000 mg/kg.