While caspases play an established role as intracellular executors of apoptosis, little is known about extracellular activities of this ubiquitously expressed family of proteases, We demonstrate here that recombinant caspase-3 retained enzymatic activity in various extracellular fluids. Experiments with cell lines, primary cells, and mice with fulminant CD95-triggered hepatitis showed that significant amounts of DEVD-aminofluoromethylcoumarine-cleaving activity, indicative of active effector caspases, were released into the medium/plasma during apoptosis, Furthermore, caspase activities were detected in liquor samples from human head trauma patients. These findings warrant closer investigation of DEVDase activity as a diagnostic marker, and of potential extracellular substrates for caspases,