To identify post-transcriptionally modulated genes at the translational level by phorbol 12-myristate 13-acetate (PMA), we investigated mRNA profiles in the polysomal and the cytoplasmic fractions of U937 cells before and after PMA stimulation using microarrays with 15017 oligonucleotide probes. Global comparison of the profiles showed that the cytoplasmic distribution of mRNAs was considerably modulated upon PMA stimulation. The results also indicate that PMA post-transcriptionally regulated at least 0.7% of detectable genes in U937 cells. Thus, besides transcriptional modulation by PMA, changes in the translational state of transcripts seem to play a critical role in PMA-induced differentiation of U937 cells.