Grimelius stain is a silver staining, allowing the demonstration of argyrophilia, that is to say the ability of cells to reduce silver salts in the presence of an external reducer. It is positive in most peripheral, normal and tumor endocrine cells.

Grimelius stain is a histological method for the detection of secretory intrcytoplasmic granules specific for carcinoid tumors. This coloration exploits the greediness of hormonal granules for silver ions that can be converted to metallic silver (argyrophilic reaction) using a reducing agent. The small silver particles are then accumulated in the secretory granules which leads to the appearance of black deposits.

With Grimelius stain, the argyrophilic granules appear in black (staining due to metallic silver particles resulting from the argyrophilic reaction) and the background and the other structures appear in yellow.