The caspase (ICE/CED-3) family of proteases has been implicated to play a crucial role in apoptosis. However, the mechanisms by which caspase activity mediates apoptosis are not fully understood. Progress in this area has been limited due to the lack of a convenient and reliable system to quantify these protease activities. In this report, we describe a quantitative assay for the activity of caspase-3, a member of the caspase family thought to mediate apoptosis in most mammalian cell types. This assay utilizes a synthetic tetrapeptide, Asp-Glu-Val-Asp (DEVD), labeled with either a fluorescent molecule, 7-amino-4-trifluoromethyl coumarin (AFC), or a colorimetric molecule, p-nitroanilide (pNA) as substrates. DEVD-dependent protease activity is assessed by detection of the free AFC or pNA cleaved from the substrates. We demonstrate the utility of the assay for rapid quantification of caspase-3 activity in the onset of apoptosis. Using the assay, we show that apoptosis induced in 32D cells under various conditions is associated with an increase in the DEVD-dependent protease activity. These studies suggest that induction of the DEVD-dependent protease activity is an indicator of apoptosis and demonstrate the utility of the assays for assessment of the role of caspase-family proteases in apoptotic cell progression.