Localization of nucleolar protein 623 in HL-60 cells under the treatment by iron chelator deferoxamine (DSF) was studied using indirect immunofluorescence. Bright nucleolar fluorescence was observed in exponentially growing control cells. The addition of DSF in the culture system resulted in timeand dose-dependent induction of protein 623 translocation from nucleoli to nucleoplasm, inhibition of cell growth, DNA and RNA synthesis. The addition of FeCI, at culture initiation completely reversed the effects of DSF. Furthermore, significant numbers of HL-60 cells could be rescued from the effects of DSF when iron was added back as late as 24 hr after exposure to DSF. Cells resumed their abilities to grow and to synthesize DNA and RNA upon the iron rescue. Protein 623, accordingly, relocated from nucleoplasm t o nucleoli. These results indicate the importance of iron for proliferation of leukemic cells and localization of protein 823 in nucleoli. Preribosomal ribonucleoprotein particles (pre-rRNPs) were extracted from isolated nucleoli of HL-60 cells and fractionated on sucrose density gradients. Protein 823 was found to be coo-localized with the pre-rRNPs as determined by ELISA assays. No such B23-associated pre-rRNPs or other pre-rRNP fractions were obtained from nucleoli of DSF-treated cells. These results suggest that one of the effects of the antiproliferative action of DSF is the inhibition of rRNA synthesis in nucleoli. Due to the lack of new synthesis of rRNA in nucleoli, protein 623 loses i t s binding target and translocates into the nucleoplasm. 623 translocation, as observed by immunofluorescence, may be a simple and rapid method for assessing inhibition of cell growth in response t o antiproliferative drugs such as deferoxamine in cancer chemotherapy.