Ruthenium red, at a concentration of 0.005%, stains extracellular but not intracellular mast cell granules. This vital stain can be used to demonstrate that after degranulation with polymyxin B sulfate many granules retained within the domain of the cell have been extruded into the extracellular space. This interpretation is confirmed by electron microscopy. Indirect evidence indicates that some extruded granules may be recovered by the cell and sequestered in an intracellular space not accessible to ruthenium red.