Granulocyte colony-stimulating factor (G-CSF) enhanced surface transferrin receptor (TfR) expression in two human myeloid leukemia cell lines, NKM-1 and NOMO-1, which possess G-CSF receptors. Radioligand-binding assay revealed that 10 ng/ml G-CSF significantly increased TfR to 186 +/- 20 and 276 +/- 38% of control for NKM-1 cells and NOMO-1 cells, respectively, in a 24-h culture. Scatchard analysis showed the increase of transferrin (Tf)-binding sites but no change in the receptor affinity. The enhanced TfR expression was not mediated either by the kinetic change of receptor cycling or by cellular iron content. Immunoprecipitation with anti-TfR antibody was used, and the increased biosynthesis of the receptor was demonstrated in G-CSF-stimulated cells. Northern blot analysis showed a 2- to 3-fold increase of TfR mRNA of NKM-1 cells cultured in medium containing Tf and G-CSF, whereas the mRNA declined without G-CSF. The effect of G-CSF on the TfR mRNA was observed within 2 h, which preceded the increase of surface TfR and the transition to the S phase of the cell cycle. G-CSF also potentiated TfR expression in freshly obtained myeloid leukemia cells. The present study shows up-regulation of TfR expression by G-CSF in myeloid leukemia cells and provides evidence that the regulation is mediated by controlling the steady-state level of the mRNA.