This paper describes a modification of a cytochemical method for the demonstration of heavy metals. The well localized precipitate in the mast cell granules, which is also present in granules that have been separated from the cell, suggest that the metals are localized in the granules. It is demonstrated that "mast cell" grown cultures do not contain precipitate. The chelating and histamine inhibiting agent 8-hydroxyquinoline produced no changes in the histochemical pattern of the mast cell granules before nor after treatment with the histamine liberator 48/80 which provokes a release of granules from the cells. These observations suggest either that the metal (zinc) is bound to the granules in such a manner that the chelating agent cannot chemically, or based on the configuration of the metal-containing molecule, reach the metal and thereby prevent its transformation to a metal suphide.