Acute toxicity studies on miocamycin (MOM), non-crystalline solid, and its metabolite Mb1 were performed in mice in the previous studies. In the present studies, we evaluated acute toxicity of Mb1 in male and female rats after single oral administration at the maximum physically applicable dose of 5,000 mg/kg. Observations were continued for 1 week after treatment. It is concluded that LD0 values of Mb1 were estimated more than 5,000 mg/kg.