The incorporation of large particulates by blood platelets is considered identical to the ingestion of bacteria by neutrophils, and is referred to as platelet phagocytosis. However, bacteria enter neutrophils in sealed vacuoles derived from the cell wall, and products deposited in the vacuoles during neutrophil degranulation are confined almost exclusively to the phagolysosomes. Products released from platelet storage organelles after uptake of foreign particles, on the other hand, are extruded to the cell exterior. The basis for this unusual difference in the phagocytic response of platelets and neutrophils has been sought in the present investigation. Combined electron microscopic and cytechemical study of platelet-latexspherule interaction revealed that platelets do not phagocytize in the usual sense. Most of the latex particles observed in platelets were lodged in channels of the open canalicular system. Channels which contained latex did not pinch off to form sealed phagocytic vacuoles, but remained open. An electron-dense tracer, lanthanum nitrate, was able to penetrate into the channels and outline the ingested latex particles. Therefore, platelets do not phagocytize latex, but sequester the spherules in preformed membranous invaginations. The persistence of open channel communication with the exterior after latex uptake may explain why platelets extrude secretory products, rather than confine them to phagolysosomes.