The activity of NF-kappa B/Rel nuclear factors is known to inhibit apoptosis in various cell types. We investigated whether the subtraction of NF-kappa B/Rel activity influenced the response of 11 AML (M1, M2 and M4) patients' cells to AraC. To this end we used a phosphorothioate double-stranded decoy oligodeoxynucleotide (ODN) carrying the NF-kappa B/Rel- consensus sequence. Cell incubation with this ODN, but not its mutated (scrambled) form used as a control, resulted in abating the NF-kappa B/Rel nuclear levels in these cells, as verified by electrophoretic mobility shift assay (EMSA) of cells' nuclear extracts. We incubated the leukemic cells with AraC (32 or 1 microM), in either the absence or presence of the decoy or the scrambled ODN, and analyzed cell apoptosis. The spontaneous cell apoptosis detectable in the absence of AraC (<25%) was not modulated by the oligonucleotide presence in cell cultures. On the other hand, in 10 of the 11 samples tested, the decoy kappa B, but not the scrambled ODN significantly (P < 0.01 in a Student's t test) enhanced cell apoptotic response to AraC. Such an effect was particularly remarkable at low AraC doses (1 microM). These findings indicate that NF-kappa B/Rel activity influences response to AraC in human primary myeloblastic cells, and suggests that the inhibition of NF-kappa B/Rel factors can improve the effect of chemotherapy in AML. Gene Therapy (2000) 7, 1234-1237.