Since highly purified rhodamine 110 (R110)-derived substrates are locked in the lactone configuration they are colorless and non-fluorescent. Cleavage of R110 peptides by caspases generates strongly fluorescent R110 that can be monitored fluorimetrically at 510-530 nm with excitation of 488 nm, the most common excitation light source used in fluorescence instruments. R110-derived caspase substrates are probably the most sensitive indicators widely used for the fluorimetric detection of various caspase activities. This R110 substrate is specific for caspase 8. Our R110-based substrates are highly purified to eliminate the trace amount of free R110 that is not detectable by HPLC, but causes significant assay background.