Ascofuranone significantly reduced serum lipid levels of rats fed with normal diet 6 hours after a single oral administration of 108 mg/kg. When the antibiotic was orally given for consecutive 10 days to normolipidemic rats, the treatment resulted in marked reduction of serum cholesterol, triglycerides, phospholipids and free fatty acids without affecting organ weight gain, serum total protein, albumin/globulin ratio and serum transaminases. Reduction was also noted with cardiac cholesterol content but liver total sterol and fecal sterol excretion were unchanged. Acute toxicity of ascofuranone is weak to mice and rats and the antibiotic did not induce hepatomegaly which is the main side effect of a positive control agent, ethyl-p-chlorophenoxyisobutyrate.