Lysosomes are intracellular organelles that contain enzymes used for the hydrolysis of waste materials and other cellular debris. The pH of the lysosome is ~ 4.5-4.8 and is optimal for these hydrolytic enzymes. The lysosomal pH gradient is maintained through vacuolar ATPases which pump protons into the lysosomes. Cayman's Lysosomal Staining Kit (Red Fluorescence) utilizes a red fluorescent dye that permeates the lysosomes based on pH gradient. Once protonated, the dye is unable to leave the lysosome resulting in enhanced fluorescence. Included in this kit is bafilomycin A1, which is an inhibitor of the vacuolar ATPase. Treatment with bafilomycin A1 results in decreased lysosomal fluorescence upon staining with the lysosomal staining reagent.