The carbohydrate content of the specific granules of atrial muscle cells has been confirmed by ultracytochemical and ultrastructural findings. The carbohydrate proteins of the granules seem to consist mainly of proteoglycans, probably with a great content of dermatan sulfate. Therefore, in mammals only the atrial muscle cells seem to retain postnatally the capability of embryonic heart muscle cells to synthesize proteoglycans. Secreted into the blood of the atria and atrial coronary vessels, they may bind lipoproteins of the blood serum, especially the atherogenic low-density lipoproteins. On the other hand, the antithrombotic effect of proteoglycans could be able to prevent a raised coagulability of the blood in the heart cavities caused by the beat of the heart cusps.