Granzyme B accumulates in an uncharacterized intracellular vesicle. HeLa target cells were incubated for 60 minutes at 37°C to allow uptake of soluble granzyme B-FITC. Cells were then immunolabeled for markers of intracellular compartments: rab5 and rab4 for early endosomes, cathepsin D and lgp120 for lysosomes, MRP for late endosomes and the Golgi apparatus, and cytochrome c for mitochondria, each with corresponding TR-conjugated secondary antisera. Labeled cells were viewed by CLSM. Scale bar is 10 μm.