The induction of DNA fragmentation by cytosine arabinoside (araC) and 2',2'-difluorodeoxycytidine (dFdC, gemcitabine) was compared in human leukemic cell lines. For both araC and dFdC this process was time- and concentration-dependent and resulted in loss of clonogenic survival of HL-60 myeloid leukemic cells. There was a marked difference in the potency between these two analogs in inducing apoptosis. A 6 h exposure to 5 microM araC was required to produce DNA laddering in HL-60 cells, whereas dFdC at a concentration 100-fold less (0.05 microM) was sufficient to produce similar results. Pre-incubation of HL-60 cells with staurosporine, a non-specific protein kinase C inhibitor, increased the level of apoptosis induced by a 3 h exposure to araC or dFdC, suggesting the possible involvement of this family of enzymes in this process. Also, dFdC was able to increase the expression of both c-jun and c-fos in Molt-3 leukemic cells with a concentration known to induce apoptosis in this cell line.