Phagocytosis in the bone marrow of rats and rabbits was studied following intravenous injections of colloidal carbon and chicken red blood cells. In both animal species the marrow response to these different foreign particles was quite similar. There was an initial aggregation and degranulation of platelets around the injected particulate matter within the marrow sinuses. Then pseudopods of marrow macrophages appeared in the sinus lumen, forming a web-like structure which trapped and phagocytosed the injected foreign material as well as the platelets. Within the phagosomes of these macrophages, the injected material and platelets were withdrawn into the parenchyma, where degradation or storage occurred. This sequence of events suggests that platelets may play an important role in marrow phagocytosis. The most active cells in marrow phagocytosis are the macrophages. The endothelial cells participated in the phagocytosis of colloidal carbon. The amount of carbon within these cells, however, was small in comparison with that trapped by macrophages. Further, the endothelial cells did not phagocytose chicken red blood cells. These results, being similar to those obtained in lymph, spleen and liver, challenge the concept of the reticuloendothelial system. The term of macrophage system is proposed as a replacement.