The human leukemia cell line K562 expresses constitutively high levels of c-myc mRNA and can be induced to differentiate along the erythroid lineage. Treatment of K562 cells with the antineoplastic drugs 1-β-d-arabinofuranosylcytosine and daunomycin causes differentiation into hemoglobin-producing cells. The differentiation process is associated with an early block of cellular proliferation occurring during the first 24 h of treatment. RNA synthesis is progressively reduced to 20 to 30% of the control levels after 3 days of exposure to the drugs. Dot and Northern blot analyses were performed to evaluate the levels of c-myc or globin mRNA during the differentiation of K562. Daunomycin and 1-β-d-arabinofuranosylcytosine, despite their distinct chemical nature, induced similar modulation of mRNA levels. Globin mRNA did not change during the first 24 h of culture and began to increase after 48 h of treatment with drugs, reflecting the kinetic of appearance of hemoglobin-producing cells. In contrast, a transient decrease of c-myc mRNA was observed after the first 24 h of drug treatment, followed by a return to normal levels of c-myc mRNA after 48 h of treatment. Thus, the expression of c-myc mRNA in K562 did not reflect their proliferative activity nor their stage of differentiation. We speculate that the transient down-regulation of c-myc mRNA may be an initial event in the erythroid differentiation of K562.