Light and electron microscope studies on the spheroid alveolar epithelial cells of the lung in mice were conducted. By the light microscopy it was found that in the case of the intraperitoneal injection of H(3)-thymidine, S(35)-DL-cysteine and S(35)-H(2)SO(4) into mice, most of siliver particles deposit in the cytoplasm and nucleus of spheroid alveolar cells. In observing this phenomenon by the electron tracers, the above tracers enter not only into the nucleus but also into mitochondria, degenerated mitochondria, and into inclusion bodies of myelin-type or lamellar microbodies in the cytoplasm of the spheroid alveolar cells. From these results it is understood that the inclusion body not clarified at present is formed by the degeneration of mitochondria. This inclusion body seems to be a kind of cytolysomes and by various enzymes contained in it, nucleic acids, proteins, mucopolysaccharides, phosphoric acid ester, and glucuronide are dissolved and finally they are reduced into the myeline figure substance as the endproduct. The endproduct is to be secreted into the alveolar lumen from the spheroid alveolar cell.