Human promyelocytic leukemia HL-60 cells were induced to undergo granulocytic differentiation by treatment with retinoic acid (RA, 10 microM, 1-5 days). The steady-state level of nucleophosmin/B23 mRNA decreased during the RA-induced differentiation. There was also decrease in the level of total cellular nucleophosmin/B23 protein during the RA-induced differentiation. Stabilization and nuclear run-on assays indicate that the decrease in nucleophosmin/B23 mRNA in RA-treated HL-60 cells was transcriptionally regulated. Unlike c-myc mRNA, there was virtually no decline of nucleophosmin/B23 mRNA during the growth arrest by serum-starvation. The decrease in nucleophosmin/B23 mRNA expression in HL-60 cells subsequent to retinoic acid treatment can thus be attributed to cellular differentiation rather than the growth arrest induced by RA. Nucleophosmin/B23 antisense oligomer treatment significantly potentiated RA-induced cellular differentiation. Results of this study suggest that nucleophosmin/B23 is one of the key elements in the down-regulation of nucleolar function for cellular differentiation.