By inhibiting the tyrosine kinase (TK) activity of Bcr-Abl, STI-571 induces differentiation and apoptosis of HL-60/Bcr-Abl (with ectopic expression of p185 Bcr-Abl) and K562 (containing endogenous expression of p210 Bcr-Abl) but not of the control HL-60 cells. Treatment with arsenic trioxide (As2O3) lowers Bcr-Abl protein levels and induces apoptosis of the Bcr-Abl-positive leukemic blasts (Blood 2000; 95: 1014). Here, we demonstrate that compared to treatment with STI-571 (0.25 to 1.0 microM) or As2O3 (0.5 to 2.0 microM) alone, combined treatment with As2O3 and STI-571 induced significantly more apoptosis of HL-60/Bcr-Abl and K562 but not HL-60/neo cells (P < 0.05). Combined treatment with As2O3 and STI-571 also resulted in greater reductions in the levels of Bcl-x(L), XIAP and Akt, and inhibition of Akt kinase activity. Co-treatment with As2O3 inhibited STI-571-induced hemoglobin, which was associated with the cleavage and downregulation of GATA-1 transcription factor involved in erythroid differentiation. These data demonstrate that a treatment strategy which combines an agent that lowers Bcr-Abl levels, eg As2O3, with an agent that inhibits Bcr-Abl TK activity, eg STI-571, can potently induce apoptosis and differentiation of Bcr-Abl-positive human leukemic cells.