DQ-BSA-RED is a red-fluorescently labeled bovine serum albumin derivative specifically designed to monitor lysosomal proteolytic activity. DQ-BSA-RED exhibits excitation and emission maxima at 590 nm and 620 nm, respectively, and contains multiple dye moieties resulting in fluorescence self-quenching until proteolytic cleavage occurs within lysosomes, upon which bright fluorescence is emitted. DQ-BSA-RED therefore enables the visualization and quantification of lysosomal functionality in live-cell assays, while inactive lysosomes fail to generate fluorescence, providing a valuable tool for studying autophagy and intracellular degradation processes.