Activation of caspases plays a central role in apoptosis. FITC-C6-LEHD-FMK provides a convenient means for sensitive detection of activated caspase-9 in living cells. FITC-LEHD-FMK is cell permeable, nontoxic, and irreversibly binds to activated caspase-9 in apoptotic cells. The FITC label allows for direct detection of activated caspases in apoptotic cells by fluorescence microscopy, flow cytometry, or fluorescence plate reader.