Abstract: Indigofera tinctoria L. (青黛) is effective against chronic granulocytic leukemia. Indirubin isolated from Indigofera tinctoria was found to be an active constituent. Be-cause of the meager content of indirubin and the increasing clinical demand for this compound, the study of synthetic indirubin was carried out in our institute. The anti-neoplastic action and toxicity of synthetic indirubin were studied in animals. Experim-ents showed that at therapeutic dosage synthetic indirubin yielded marked inhibition of Lewis lung carcinoma and Walker carcinosarcoma 256, but the inhibition on the gro-wth of L₇₂₁₂ leukemia and P₃₈₈ leukemia in mice was not significant. Subacute toxicity tests showed that in rats given synthetic indirubin 100～400 mg/kg orally for 30 cons-ecutive days, no significant untoward effects on the number of leukocytes, liver (BSP retention) and renal (blood NPN) functions were observed. In dogs given synthetic indirubin, 20～40 mg/kg orally for 1 to 3 consecutive months, no significant abnormal-ity was observed. But in one dog synthetic indirubin induced severe anorexia and diar-rhea when the dosage was increased to 80 mg/kg/day. (Two weeks after withdrawal of the drug the GI reaction disappeared.) However, no bone marrow inhibition was obser-ved at the dosage of 40 mg/kg daily for 1 month and 80 mg/kg daily for 26 consecu-tive days.