K562 is a Philadelphia (Ph) chromosome-positive chronic myelogenous leukemia (CML) blast crisis cell line representing a pluripotent precursor cell. At the molecular level, K562 cells express high levels of the aberrant bcr-abl product, p210bcr-abl, believed to be critical to the pathogenesis of CML. The authors demonstrate that exposure of K562 cells to hemin causes a state of partial, reversible erythroid maturation, accompanied by a marked decrease in p210bcr-abl. The change in bcr-abl expression may be mediated at the translational level since steady state amounts and enzymatic activity of the bcr-abl protein are reduced whereas bcr-abl mRNA levels are unaltered. The decrease in p210bcr-abl phosphokinase enzymatic activity can be detected within 2 hours after addition of hemin to the culture media, indicating that changes in expression of this oncogene probably occur before or concurrent with differentiation. No change in bcr-abl protein occurred in a CML cell line (KBM-5) which did not undergo differentiation after exposure to hemin, consistent with a direct relationship between altered p210bcr-abl expression and hemin-induced erythroid differentiation. Importantly, the marked diminution in bcr-abl protein was not associated with a disruption in K562 growth rates, indicating that the proliferative capacity of these cells may be independent of the bcr-abl product. In contrast to hemin, cytosine arabinoside (Ara-C) caused terminal erythroid differentiation of K562 cells, characterized by irreversible hemoglobin accumulation and cytostasis; and no change in bcr-abl protein expression was observed. The distinct effects of Ara-C and hemin could reflect the existence of pleiotropic differentiation pathways. Both Ara-C and hemin-exposed cells showed a decrease in c-myc and c-myb transcripts, suggesting that altered levels of these proto-oncogenes may be associated with erythroid maturation, regardless of the rate of cell division. K562 cells provide a useful model for analyzing the interaction between oncogene expression and CML cell growth and differentiation.