Mutation of the p53 tumor suppressor gene is recognized to be a key event in the development of the highly aggressive behavior of undifferentiated or anaplastic thyroid carcinomas. Attempts to treat these carcinomas with p53 gene therapy have, however, been largely unsuccessful. Since epigenetic changes such as histone deacetylation are associated with loss of thyroid differentiation, we have evaluated the potential of combining p53 gene therapy with exposure to the histone deacetylase inhibitor (HDAC-1), depsipeptide. We used two carcinoma cell lines: FRO cells that express very low levels of p53 and WRO cells producing a dominant negative p53. A p53 response element luciferase assay showed that stimulation of p53 transcriptional activity by the combined treatment with the HDAC-1 and p53 was 10 to 100 times greater than with p53 alone. Western blot analysis demonstrated that the HDAC-1 increased the expression of acetylated histones, as well as of p21(cip1/waf1), but did not affect levels of total histone and endogenous p53. The combined treatment was much more effective than either treatment alone in inhibiting the growth of both cell lines, and flow cytometric analysis suggested that this was due to an increase in the sub-G1 apoptotic population. Our findings indicate that HDAC-1 enhances apoptotic killing by p53 transfer, and suggest that this combination strategy may be useful in treating undifferentiated thyroid carcinomas.