An electron microscope examination of the liver of the crab-eating monkey revealed small crystalloids occurring occasionally in the thicker portion of the cytoplasmic extension of the sinusoidal endothelium. They were uniformly encased in a membrane sac which was mostly smooth-surfaced but was at several points continuous to ribosome-studded cisternae of the RER. The crystalloids were mostly polygonal in configuration and were classified into three types. Type I crystalloids, according to the grade of the complexity of their composition, represented the simplest, or original, form and were composed purely of a compact bundle of tubules measuring about 300Å in diameter. Types II and III crystalloids were composed of tubules and an electron lucent matrix. In type II crystalloids, the tubules were embedded parallel to one another in two sets of matrix layers which crossed each other at a right angle, while in type III, the matrix layers embedding the tubules cut each other at about 70°. The crystalloids are presumed to have developed from a substance synthesized in the cisternae of the RER in the sinusoidal endothelial cell and their investigation may aid in elucidating the proteinic products of this RER-rich cell which have thus far remainedunder dispute.