Altering chromatin structure by blocking histone deacetylase activity with specific inhibitors such as trichostatin A can result in an up-regulation of gene expression. In this report, however, we show that expression of the ETS domain transcription factor PU. 1 is down-regulated in cells following the addition of trichostatin A. The loss of PU. 1 is seen at both the mRNA and protein levels in multiple cell lines and is reversible following removal of the drug. More importantly, we show that the loss of PU. 1 results in a loss of PU. 1 target gene expression, including CD11b, c-fms, Toll-like receptor 4, and scavenger receptor. Chromatin immunoprecipitation analysis of cells treated with trichostatin A showed a significant increase in the acetylation of histone H4, but not histone H3, across approximately 650 bp of the PU. 1 promoter region. Our data suggest that the consequences of using drugs that inhibit histone deacetylase activity may be a loss of blood cell development and/or function due to a block in PU. 1 gene expression.