The antitumor activities of various extracts from the roots of Agrimonia pilosa LEDEB. were studied. Each extract was given intraperitoneally to mice once at 4 d before the intraperitoneal inoculation of mouse mammary carcinoma MM2 cells, and the non-sugar fractions with median polarity showed antitumor activity. Agrimoniin was isolated from the antitumor active fractions. Agrimoniin itself showed antitumor activity when given as a pre- or posttreatment ; a single dose of 10 to 30 mg/kg of agrimoniin resulted in almost complete rejection of the tumor by the test mice. Agrimoniin also possessed a high degree of cytotoxicity but this activity was significantly reduced by the addition of serum to the culture medium.