An electron microscope study was made to elucidate the manner in which reticular fibers form in the developing spleen of the chick embryo. On the 7th day of incubation, the splenic reticulum consisted of dendritic reticular cells without reticular fibers. From the 9th day onward, the intercellular gaps between processes of adjacent reticular cells were expanded, becoming occupied by fine flocculent materials. With the growth of the embryos, these materials were aggregated into felt-like bands, exhibiting the characteristics of mature reticular fibers. In the interstices of the reticulum, there appeared microfibrils of 8-30 nm in diameter. The reticular cells possessed numerous free ribosomes and polysomes, well-developed Golgi apparatuses, dilated rough-surfaced endoplasmic reticula, and many coated vesicles. The coated vesicles contained fine flocculent materials as presumable fiber precursors. The vesicles were enlarged as they approached the cell periphery, where they appeared to discharge their contents by reverse pinocytosis. some vesicles close to the cell membrane contained ruthenium red-positive materials which were considered to be glycosaminoglycan in nature and to compose the matrix of fibers. These materials also seemed to be secreted by reverse pinocytosis, being densely accumulated in the intercellular gaps. The differentiation of fibrous proteins into felt-like reticular fibers or striated microfibrils seems, therefore, to be intimately associated with the amount of glycosaminoglycan.