The levels of iron-responsive element-binding protein (IRE-BP I) mRNA throughout the course of erythroid differentiation were investigated in several lines of murine erythroleukemia (MEL) cells (Friend 745, 707 and Fw cells). Fw cells are not inducible for ferrochelatase activity and heme synthesis. Cytoplasmic ferrochelatase mRNA and transferrin receptor (TfR) mRNA levels are only insignificantly increased in Fw cells after induction. We have found increased levels of (IRE-BP 1) mRNA during erythroid differentiation of MEL cells of all lines investigated. Run-on transcription reactions using isolated nuclei from Friend 707 cells showed increased (IRE-BP 1) gene transcription following induction of erythroid differentiation with 5 mmol hexamethylenebisacetamide (HMBA). The increase in (IRE-BP 1) gene transcription is only about 2-fold in comparison with 8-fold increase in the level of (IRE-BP 1) mRNA during 96 hours of Friend 707 cells induction. These findings indicate that the stability of (IRE-BP 1) mRNA might also play a role in the increase of (IRE-BP 1) mRNA levels after Friend 707 cells induction. The possible role of increased (IRE-BP) mRNA levels in the elevation of TfR numbers during erythroid differentiation is discussed.