We examined the epigenetic mechanisms involved in human T-cell lymphotropic virus type 1 (HTLV-1) Tax expression. Blockade of histone deacetylation with trichostatin A (TSA) resulted in Tax upregulation. Using a chromatin immunoprecipitation (ChIP) assay, we verified local histone hyperacetylation at the HTLV-1 LTR in response to TSA. In agreement, HDAC3 transfection led to reductions in both Tax expression and histone acetylation. HDAC3 mutations and deletions spanning the catalytic site had variable ability to repress Tax, but HDAC activity was not essential for repression. Immunoprecipitation studies revealed that Tax co-exists in a complex containing both histone deacetylase 1 (HDAC1) and 3 (HDAC3). Our results suggest that HDACs may actively participate in the repression of HTLV-1 Tax transcription.