Deferoxamine (DFO), a widely used therapeutic iron chelator, was found to inhibit proliferation of the promyelocytic leukemia cell line HL-60 in a dose-dependent fashion when tested in a clonogenic assay at concentrations ranging from 1.0 to 10.0 microM. Cytosine arabinoside, methotrexate, and daunorubicin also produced dose-dependent inhibition of HL-60 colony growth when tested singly in vitro. When DFO, 1.0 microM, was included with each agent in dose-response studies, a synergistic enhancement of the antiproliferative effects was observed. This synergism probably results from a DFO-induced decrease in intracellular levels of deoxyribonucleoside triphosphates and an inhibition of the cells at the early S-phase of cell cycle. Our data suggest that DFO has potential as an adjunctive antileukemic agent.