Ready-to-use colorimetric substrate for caspase-5 and related caspases that recognize the amino acid sequence WEHD. Caspase-5 and related caspase activity can be quantified by spectrophotometric detection of free pNA (l = 400 nm) after cleaved from the peptide substrate WEHD-pNA, using a spectrophotometer or multi-well plate reader. The ready-to-use caspase substrate provides an economic alternative for researchers who perform large volume of caspase assays.