Analytical subcellular fractionation is playing an increasingly important role in proteomic studies to identify and validate components of cellular organelles. For lysosomes, definitive studies in this area have been restricted to rodent tissues due to technical constraints. Our goal was to design a quantitative assay that would allow clear demonstration of lysosomal localization in cultured human cells. We found that culturing HepG2 (human hepatocellular carcinoma) cells in progesterone-containing medium elicited an extensive shift in the buoyant density of lysosomes as measured by isopycnic centrifugation in sucrose density gradients. The density of other organelles remained essentially unchanged; thus, this shift represents a specific test for lysosomal localization. Progesterone treatment of a variety of other cultured cells also elicited a shift in lysosome density. This approach should represent a valuable tool for identification and validation of both luminal and membrane lysosomal proteins.