The treatment of ICR mice with i.p. injections (0.14 g/kg/day) of the extract of Capsella bursa-pastoris herb (Cruciferae) caused 50 to 80% inhibition of the solid growth of Ehrlich tumor cells that had been inoculated into the s.c. tissue of the animals. The tumor lumps in the treated mice showed multifocal necroses and the infiltartion of host fibrous tissue cells. Experiments were also performed to isolate and identify the active component for the antitumor action, and an acidic substance was isolated in crystalline form from the herb extract. This acidic substance was identified as fumaric acid and was effective in inhibiting the growth of Ehrlich solid tumor at a dose of 10 mg/kg/day. The 50% lethal dose (i.p.) of this acid was 266 mg/kg.