Six indirubin derivatives have been synthesized and their inhibitory effects on the growth of HL-60 human promyelocytic leukemia cells investigated. Cell viability was determined using the trypan blue exclusion method. Indirubin-3'-oxime (I-1) inhibited the growth of HL-60 cells with a GI50 value of 36.6 microM, whereas I-0, I-2, I-3, I-4 and I-6 showed only weak cytotoxic activities against HL-60 cancer cells with GI50 values in the range of 97.3 to over 100 microM. These results indicate that indirubin derivatives might be useful candidate agents for exploring potential antileukemic drugs.