Clearance of foreign materials from the hemocoel of decapod crustaceans involves several distinct kinds of cells. Proteins, and possibly viruses below 30 nm diameter, are known to be removed by the branchial podocytes, which are specialized pinocytotic nephrocytes located in the gills. These cells, and podocytes of the antennal gland (excretory organ) which may also be of importance in clearance, are similar to podocytes of Bowman's capsule of the vertebrate kidney. Phagocytic clearance of particulate material (with possible exception of the small viruses) is accomplished by three kinds of cells, one free and the other two fixed to tissues facing hemal spaces. The hemocytes, free cells suspended in the hemolymph, are the most abundant and generalized of these phagocytic cells. Phagocytic reserve cells are applied to myofibers in the heart of penaeid and palaemonid shrimp, and probably the same as reserve cells found throughout the hemocoel of all decapods. They are minimally involved in clearance of injected carbon and carmine, but are not known to phagocytize naturally occurring particles such as bacteria or viruses. The fixed phagocytes are very important in the clearance of some substances. They are located on the exterior surfaces of arterioles in hemal spaces of the hepatopancreas. They are highly specialized cells designed to sequester large amounts of various types of particulate matter, not only through phagocytosis but by trapping and retaining vast numbers of particles within a net- or sievelike layer of granular material which surrounds the free surface of the fixed phagocyte. Derivation, morphology, development, and function of the above cell types are described and discussed.