(Z-Asp-Glu-Val-Asp)2-Rhodamine 110, a highly sensitive and photostable fluorogenic substrate, is used to determine intracellular caspase-3 activity. Fluorophore Rhodamine 110 (R110) can be detected at an emission wavelength of 535 nm using an excitation wavelength of 485 nm. As with (H-Asp-Glu-Val-Asp)2-Rhodamine 110, the fluorescence intensity can be measured by flow cytometry, microwell plate reader, or fluorescence microscopy.