Cytosine arabinoside (ara-C) and 2',2'-difluorodeoxycytidine (Gem) were compared in leukemia cells, with Gem being more potent than ara-C. Gem was combined with hexadecylphosphocholine (HPC) or erucylphospho-N,N,N-trimethylpropanolamine (ErPC(3)) in resistant CML cells. Supra-additive effects were seen in K-562 cells after concomitant and sequential exposure of Gem followed by HPC. The reverse sequence resulted in antagonism. Both effects were more significant when HPC was exchanged for ErPC(3). Gem or HPC failed to induce DNA laddering in K-562 cells, but apoptotic signals were transferred by the Gem-exposed SKW-3 cytosolic fraction to K-562 nuclei. HPC did not increase the clastogenicity of Gem and counteracted its mitotic inhibition in murine bone marrow. Thus, the combination of Gem and an alkylphosphocholine is advantageous in terms of their complementary mode of action, resulting in increased cytotoxicity and lowered myelotoxicity.