Ready-to-use colorimetric substrate for FLICE/caspase-8 and related caspases that recognize the amino acid sequence IETD. The sequence IETD is based on caspase-8 cleavage site in CPP32/Caspase-3 proenzyme. FLICE and related caspase activity can be quantified by spectrophotometric detection of free pNA (l= 400 nm) after cleavage from the peptide substrate IETD-pNA, using a spectrophotometer or multi-well plate reader.