Proliferation of HL-60 and MOLT4 leukemia cells was inhibited by a c-myb antisense oligonucleotide (ASO) in the presence of a DNA uptake-stimulating protein (DNA uptake-stimulating factor, DUSF). The inhibitory effect was very mild in the absence of DUSF. Sense oligonucleotides or DUSF, alone or in combination, were found to be ineffective. Cellular expression of the c-myb protein was significantly more inhibited by the c-myb ASO in the presence than in the absence of DUSF. In the presence of DUSF, c-myb protein practically disappeared from the nuclei of HL-60 and MOLT4 cells treated with the ASO. Thus, DUSF appears to effectively stimulate the uptake of c-myb ASO into tumor cells in vitro, augmenting its antiproliferative effect by decreasing c-myb expression.