Calpain proteases have been extensively studied over the past 20 years but substrate specificity and activity studies have been limited to investigations using isolated calpain proteases in vitro and in the presence of calcium concentrations beyond the physiologic range found in normal cells (1–3). Intracellular autolytic activation, interactions with small acidic phospholipids, and interactions with the calpain inhibitor protein calpastatin all modulate physiologic and pathophysiologic calpain activity (1–3). Thus, the development of calpain assays for use in intact cells is critical to the study of the normal and pathological activity of these proteases.