An internally quenched fluorogenic substrate (Ab/Em =328/393 nm) for cathepsins D and E and not for B, H or L, obtained from the hepatopancreas (liver) of the Japanese common squid (Todarodes pacificus). The cleavage occurs at the Phe-Phe amide bond resulting in enhanced fluorescence and is used in screening cathepsin D and E inhibitors and for determining cathepsin D and E activity in tissue cell extracts.