The TriboTM HDAC Colorimetric Assay Kit provides a convenient, sensitive, and simple approach to quantify HDAC activity from various sources of enzyme using a colorimetric substrate. In HDAC assay, an acetylated substrate is incubated with HDAC-containing samples. Deacetylation of substrate sensitizes it in presence of HDAC developer, generates the colorimetric substrate, which can be detected at OD395nm. The substrate included in the kit is cell-permeable, and the assay can measure HDAC activity directly in cell culture in a 96-well plate without a time-consuming cell extraction step. The kit also can be used for HTS of HDAC inhibitors with extracts or purified enzymes.