Elevation in the cytosolic Ca²⁺ concentration triggers exocytosis of lysosomes in many cell types. This chapter describes a method to detect lysosomal exocytosis in mammalian cells, which takes advantage of the presence of an abundant glycoprotein, Lamp1, on the membrane of lysosomes. Lamp1 is a transmembrane protein with a large, heavily glycosylated region that faces the lumen of lysosomes. When lysosomes fuse with the plasma membrane, epitopes present on the luminal domain of Lamp1 are exposed on the cell surface. The Lamp1 luminal epitopes can then be detected on the surface of live, unfixed cells using highly specific monoclonal antibodies and fluorescence microscopy. The main advantage of this method is its sensitivity, and the fact that it provides spatial information on lysosomal exocytosis at the single cell level.