Indirubin is the ac ti ve ingredi ent of a traditional Chinese herbal medicine, Danggui Longhui Wan, used for t he t reatment of chronic myelocytic leukemia Here, we report that novel indirubin ,- 11‘ ivative‘ 5’ - nitro-indirubinoxime (5’ NIO) , has potent anti- proliferative activity on va rious human cancel‘ cells and oncogenic RK3E- ras rat kidney cells with ha lf- inhibi tory concentrati ons (1C50) ra nging from 1- 12 M, Treatment with indirubin derivative induced the activation 01' caspase 7 rollowed by apoptos is in RK3E- ras cells. lndirubin derivative showed strong anti-tumor activity in rat solid and oral tUll10r models , Direct inj ection of indirubin deri vative every other day for 10 days induced signifi cant inhi bition of tumor growth in Sprague-Dawley rats bearing RK3E- ras-induced tumors Histologically. t reatment with indiru bin de ri vative caused s ignifi cant inhi bit ion of tumor formation with increased apoptosis and decreased tumor cell prolife ration, These f indings provide the potent ial va lue of indirubin deri vative a s a novel candidate for ant i-tumor agents