n-Butyrate inhibits the growth of colon cancer cell lines. In the HCT 116 cell line, butyrate-induced growth inhibition is almost fully reversible, whereas in the VACO 5 cell line, a subpopulation undergoes apoptosis within 30 hr of treatment with butyrate. Concurrent treatment of VACO 5 cells with butyrate and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) accelerates and increases the incidence of cell death to nearly 100% of the population, whereas HCT 116 cells largely remain alive during treatment with this combination. The action of butyrate as an inhibitor of histone deacetylase was assessed in these cell lines by examining extracted core histones for their electrophoretic mobility in Triton/acid/urea gels. The concentrations of butyrate that were effective for inducing apoptosis were similar to the concentrations that caused hyperacetylation of core histones in the VACO 5 cell line. Furthermore, an examination of other carboxylic acids for induction of apoptosis revealed a rank order that corresponded to the order of potency in causing hyperacetylation of core histones. Specifically, the active acids were 3-5 carbons in length and lacked substitution at the 2-position. Isovaleric and propionic acids, in particular, proved to be effective inducers of both hyperacetylation and apoptosis at 5 mM concentrations, a finding of potential relevance to the unusual pancytopenia occurring after acidotic episodes in isovaleric and propionic acidemias. The duration of butyrate treatment required for chromatin fragmentation (10-20 hr) corresponded to the time required for histone H4 to become predominantly tetraacetylated. Furthermore, trichostatin A, a structurally dissimilar inhibitor of histone deacetylase, mimicked butyrate-induced apoptosis of VACO 5 cells and growth inhibition of HCT 116 cells. The dramatic enhancement of VACO 5 cell death by TPA, and the high level resistance of HCT 116 cells to butyrate were not evident from histone acetylation determinations. Thus, applications of butyrate for cytoreduction therapy will benefit from pharmacodynamic assessment of histone acetylation, but will require additional work to predict susceptibility to butyrate-induced death.