Activation of ICE-family proteases/caspases initiates apoptosis in mammalian cells. Chemicon's Caspase-9 Colorimetric Activity Assay Kits provide a simple and convenient means for assaying the activity of caspases that recognize the LEHD. The assay is based on spectophotometric detection of the chromophore p-nitroaniline (pNA) after cleavage from the labeled substrate LEHD-pNA. The free pNA can be quantified using a spectrophotometer or a microtiter plate reader at 405 nm. Comparison of the absorbance of pNA from an apoptotic sample with an uninduced control allows determination of the fold increase in caspase-9 activity.