To verify the reported differentiation/maturation-inducing effects of cytosine-beta-D-arabinofuranoside (ara-C, Sigma Chemical Corp, St Louis) on hematopoietic cells, we studied the morphologic changes and patterns of proto-oncogene expression in HL-60 cells that had been exposed to the drug. At the 1 X 10(-7) mol/L concentration of ara-C, approximately 10% of HL-60 cells became nitroblue tetrazolium (NBT) reduction test positive after four days exposure. However, no cells became nonspecific esterase-positive during the culture period. Doses less than 1 X 10(-8) mol/L had virtually no effect on maturation and proliferation of the target cells while doses greater than 1 X 10(-6) mol/L were lethal to HL-60 cells. Cells treated with 1 X 10(-7) mol/L and 1 X 10(-9) mol/L ara-C did not evidence any of the changes in c-myc, c-myb, c-fos, or c-fes that are noted when dimethyl sulfoxide (DMSO) or 12-0-tetradecanoyl phorbol 13-acetate (TPA, Sigma Chemical Corp, St Louis) are used to induce the differentiation of HL-60 cells. In both doses, temporary decreases of the S-phase specific and proliferation related histone H3 gene expression occurred. This phenomenon may be related to an increase in the tendency of these cells to spontaneously differentiate. Because of the possibility of drug inactivation during culture and because HL-60 cells have already matured to the promyelocytic stage, these kinds of experimental systems may be inadequate in in vitro models for low-dose ara-C therapy.