The ability and capacity of various tissues in platy Xiphophorus maculatus L. to take up horse-spleen ferritin injected into the blood stream are described. Ferritin was injected intraperioneally, and the cellular uptake was demonstrated as Prussian blue precipitations in tissues treated with acid ferrocyanide solutions. Ferritin was detected within the heart endocardial cells and macrophages in the trunk kidney and spleen, 1/4 h after the injection, i.e. foreign ferritin was taken up very rapidly by these cells. When the time elapsed between the ferritin injection and sacrifice exceeded 6 h, these cells, and also macrophages in the gill and intestine, were almost completely filled with ferritin. At these stages, however, the amounts of Prussian blue precipitations per volume unit of the tissue were much larger in the heart than in the other organs studied in the present work, i.e. the endocardial tissue seems to play an important role in the clearance of the blood circulation in this species. We suggest that this tissue in platy is specialized to endocytose waste and foreign macromolecules, including pathogenic particles, from the blood stream. The eosinophilic and neutrophilic granulocytes do not appear to take up foreign ferritin, i.e. these cells may play no endocytotic role in the clearance of foreign macromolecules in platy.