The combined effect of ZOL and imatinib mesylate on leukemic cell proliferation. CI is plotted as a function of the fraction affected (Fa), which represents the percentage of growth inhibition and was evaluated using the modified MTT assay (0.5 = 50%). This allows the combination of multiple equipotent drug concentrations to be analyzed for synergistic (CI < 1), additive (CI = 1), or antagonistic (CI > 1) effects. Panel A represents the result in BV173; panel B represents the result in K562; and panel C represents the result in HL60. Data points show the means of 3 independent experiments. Bars represent 1 SD.