Diallyl disulfide (DADS) is a naturally occurring organosulfur compound, from garlic, which exerts pleiotropic biological effects. In rodents, DADS inhibits colon chemically induced carcinogenesis. DADS anti-promoting effect may partly result from its ability to inhibit tumoral cell proliferation in vivo and in vitro. As far as DADS may modulate the expression of a subset of genes, we investigated DADS effect on histone acetylation, in two human colon tumor cell lines. Our study demonstrates that in Caco-2 and HT-29 cells treated for 6 h, 200 microM DADS increases histone H3 acetylation (x2 and x1.4, respectively). In Caco-2 cells, we also observed histone H4 hyperacetylation, preferentially at the lysine residues 12 and 16. We explored the effects of DADS and one of its metabolites, allyl mercaptan (AM), on histone deacetylase (HDAC) activity: using nuclear extracts of Caco-2 cells, 200 microM DADS decreased HDAC activity by 29% and AM at the same concentration was more efficient (92% inhibition). We also observed that DADS induced an increase in p21(waf1/cip1) expression, at mRNA and protein levels, in both cell lines. This effect was associated with an accumulation of cells in the G2 phase of the cell cycle. Our results suggest that in Caco-2 and HT-29 cells, DADS could inhibit cell proliferation through the inhibition of HDAC activity, histone hyperacetylation and increase in p21(waf1/cip1) expression. The present study provides evidence for cellular and molecular responses triggered by DADS that could be linked to its effect on histone acetylation and play a role in its protective properties on colon carcinogenesis.