The interplay between the acetylation and deacetylation activities within the cell has been postulated to be a mechanism by which the cell regulates expression from genes at the level of chromatin. We have examined the expression pattern of the human histone deacetylase gene HDAC1 and the cyclin dependent kinase inhibitor p57Kip2 in the hepatocellular carcinoma cell line Hep 3B. HDAC1 expression was elevated at low cell densities, but once a critical threshold point in cell density was attained, expression was reduced to very low levels. Treatment of the cells with trichostatin A (TSA), a potent inhibitor of histone deacetylases, was found to affect expression. p57Kip2 was found to be downregulated by TSA, whereas HDAC1 was upregulated. These effects were found to be cell density dependent. The results suggest that HDAC1 plays a role in its own regulation, and that investigations using TSA should be carried out when cells grow exponentially.