SRB-VAD is a red fluorescent cell-permeable polycaspase inhibitor to target caspases 1, 2, 3, 6, 8, 9, or 10. Once inside the cell, the inhibitor binds covalently to the active caspase, which produces a green fluorescence. When added to a population of cells, the SRB-VAD-FMK probe enters each cell and covalently binds to a reactive cysteine residue that resides on the large subunit of the active caspase heterodimer, thereby inhibiting further enzymatic activity. The bound labeled reagent is retained within the cell, while any unbound reagent will diffuse out of the cell and is washed away. The green fluorescent signal is a direct measure of the amount of active caspase present in the cell at the time the reagent was added. Cells that contain the bound labeled reagent can be analyzed by 96-well plate-based fluorometry, fluorescence microscopy, or flow cytometry. The probe has the spectral properties similar to Cy3 and TRITC, thus the filter set of Cy3 and TRITC can be conveniently used with a flow cytometer or microscope.