Imatinib mesylate (Gleevec, STI571) is a specific inhibitor of the Bcr/Abl fusion tyrosine kinase that exhibits potent antileukemic effects in chronic myelogenous leukemia. Bcr/Abl-positive K562 and Bcr/Abl-negative HL-60 human leukemia cells were used to investigate the effect of PRI-2191, a calcitriol analog, on the biological effects of imatinib combined with other anticancer drugs. The results show that PRI-2191 enhances the antiproliferative effect of imatinib on HL-60 cells. When these two agents together are applied with either docetaxel or cisplatin, but not with idarubicin, the antiproliferative effect could still be enhanced. Moreover, when the interaction between the chemotherapy agents was antagonistic or additive, PRI-2191 could even shift it to synergism. This effect correlated with an accumulation of HL-60 cells in the G0/G1 phase of the cell cycle and a decrease in the percentage of cells in the G2/M and S stage in the ternary combinations used. PRI-2191 did not influence apoptosis induced by imatinib alone or in ternary combinations with all the chemotherapy agents used. These results may suggest that the stronger antiproliferative effect of the combined treatment with PRI-2191 on HL-60 cells is related to cell cycle arrest rather than to the induction of apoptosis.