Electron-microscopic studies in the spinal cord of neonatal rats show, though rarely, the existence of amoeboid microglial cells. The cells are distributed singly and are found only in the spinal grey matter. They are characterized by having a round, prominent nucleus with peripheral chromatin clumps. The cytoplasm is endowed with a well-developed Golgi complex, and isolated profiles of rough endoplasmic reticulum. One striking feature of the cytoplasm is the presence of abundant lysosome-like granules. The cytoplasm, which is of moderate density, may be vacuolated. The surface of the cells often sends out pseudopodium-like processes. These, together with the presence of occasional phagosomes, indicate that the amoeboid cells are phagocytic. Thus, it is possible to generalize from the present and earlier findings that amoeboid microglial cells are normal cellular constituents in the maturing central nervous system and their temporary existence in the neonatal stage indicates the necessity of these cells for subsequent maturation of the nervous tissue.