Apoptosis can be mediated by mechanisms other than the traditional caspase-mediated cleavage cascade. There is growing recognition that alternative proteolytic enzymes such as the lysosomal cathepsin proteases may initiate or propagate proapoptotic signals, but it is currently unclear how cathepsins mediate apoptosis. Cathepsins are lysosomal enzymes that are also used as sensitive markers in various toxicological investigations. BioVision’s newly developed Cathepsin Activity Assay kits are fluorescence-based assays that utilize the preferred substrate sequence for each cathepsin, labeled with AFC (amino-4-trifluoromethyl coumarin). Cell lysates or other samples that contain cathepsins will cleave the synthetic substrate to release free AFC. The released free AFC can easily be quantified using a fluorometer or fluorescence plate reader. The cathepsin assays are simple, straightforward, and can be adapted to 96-well plate assays. Assay conditions have been optimized to obtain the maximal activity. Active cathepsins, cathepsin inhibitors and cathepsin antibodies are also available.