Shikonin, a naphthoquinone contained in the root of Lithospermum officinalis var. erythrorhizon MAX. and Macrotomia euchroma (ROYLE) PAULS (Boraginaceae), showed a high antitumor activity against the ascites cells of Sarcoma 180. Shikonin completely inhibited the tumor growth at a dose of 5-10 (mg/kg/day). At a higher dose (>15 mg/kg/day) shikonin showed'toxicity, whereas at a lower dose (1 mg/kg/day) it was inactive. Four shikonin derivatives and the chromatographic fractions of the plant extracts were also tested and they showed similar antitumor activities against S-180.