Abstract: Armillarisin A (3-acetyl-5-hydroxymethyl-7-hydroxycoumarin), a new coumarin derivative, was isolated from the ethanol extracts of culture medium of a fungus, Armillariella tabescens (Scop. ex Fr.) Sing. The drug when given intraduodenally or intramuscularly to rats at dosage of 30 mg/kg, or intravenously to dogs at dosages of 5 and 10 mg/kg induced a marked increase in bile secretion by the liver. When administered intravenously to dogs at dosages of 5 and 10 mg/kg, Armillarisin A reduced the tone of the terminal bile duct. The acute intraperitoneal and oral LD₅₀ for Armillarisin A in mice was found to be 980 mg/kg and>5000 mg/kg, respectively. When Armillarisin A was given intramuscularly at dosages of 1.6 and 3.2 mg/kg/day to rats, 8 mg/kg/day to dogs for three months, no adverse effect attributable to the drug was observed.