Human leukemia K562 and HeLa cells were treated with daunomycin (DA) for 1-4 hr. With the indirect immunofluorescence technique, we observed that the nucleolar protein nucleophosmin/B23 (NPM) shifted its location from the nucleolus to the nucleoplasm (NPM-translocation). The degree of NPM-translocation was determined by the relative immunofluorescent intensity in the nucleoli vs the nucleoplasm (defined as localization index, LI). We found that NPM-translocation, as determined by the decrease of LI, correlates with cytotoxicity. The degrees of NPM-translocation, chromatin condensation, and DNA fragmentation in HeLa cells were determined after treatment with 0.1, 0.5 and 1 microg/mL DA for 1 hr. We found that NPM-translocation (LI < 2.5) was observed in cells during the treatment with 0.5 and 1 but not with 0.1 microg/mL DA. Also, cells treated with 1 microg/mL remained in an NPM-translocated state for a longer time (5-6 hr) than those cells treated with 0.5 microg/mL (1-2 hr). Cells treated with 0.5 and 1 microg/mL DA showed increased levels of chromatin condensation beginning at 5 hr after the drug treatment. The number of cells with condensed chromatin increased with both time and drug concentration. No cells with condensed chromatin were observed in samples treated with 0.1 microg/mL DA, which also showed no significant NPM-translocation. Similar results were observed for induction of DNA fragmentation. We found that the drug concentration required for induction of DNA fragmentation and chromatin condensation coincided with the drug concentration required for NPM-translocation. Taken together, these results indicate that NPM-translocation correlates with apoptosis induced by daunomycin.