BACKGROUND: The efficacy of DNA vaccines to date has overall been disappointing, especially in large animal models and in humans, which might be partially explained by the chromatinization of the administered foreign DNA. Trichostatin-A (TSA) is a specific inhibitor for histone deacetylase (HDAC) and a gene expression enhancer in in vitro DNA viral infection, including herpes simplex type 1 (HSV-1) and cytomegalovirus (CMV). OBJECTIVES: We sought to determine if increasing antigen expression of DNA vaccines through inhibition of HDAC-induced chromatin silencing using TSA would enhance DNA vaccine efficacy in vivo. STUDY DESIGN: A luciferase assay was used to detect effects of TSA on different promoters in vitro. The effects of TSA on DNA vaccination of mice were determined by neutralization assays for antibody production and interleukin staining for detection of specific T cell responses. RESULTS: Mice receiving TSA in combination with a DNA vaccine exhibited higher antibody responses to the vaccine than mice not given TSA. Co-administration of TSA also enhanced specific CD8 T cells response. CONCLUSION: Drugs such as TSA that reduce initial gene silencing by preventing histone deacetylation can increase immune responses to DNA vaccination.