Histone deacetylases (HDACs) are important regulators of transcription, and their inhibitors are a promising class of anticancer agents. The methods for the determination of HDAC activity and its inhibition that are currently available suffer from various drawbacks, such as animal testing, radioactive substrates, or limited throughput. Therefore, a fast nonisotopic method for the measurement of HDAC activity is highly desirable. The authors present such an assay that relies on the fluorescent HDAC substrate developed previously in their group. After incubation of the substrate with the enzyme, a derivatization leads to efficient fluorescence quenching in the deacetylated metabolite. Thus, only the fluorescence emitted by the remaining substrate is detected, which allows for a convenient detection of HDAC activity in a homogeneous format that can be performed on multiwell plate readers. This procedure, called HDASH (histone deacetylase assay-homogeneous), should be a valuable tool in transcriptional research and especially drug discovery.