Abstract: N-methylisoindigotin (Dian Ⅲ) is an analogue of indirubin. When given orally it exhibited significant inhibition against Lewis lung cancinoma and Walker carcinosarcoma 256. However, the effect of Dian Ⅲ on the growth of L₁₂₁₀ lymphocytic leukemia was insignificant. Dian Ⅲ was shown to be 2 times more toxic than indirubin in dogs and exhibited an inhibitory action against Lewis lung carcinoma 8 times as great. Dian Ⅲ given orally or intraperitoneally markedly inhibited the incorporation of ³H-TdR into DNA of Walker 256 tumor cells in rats.