A case of lung cancer with elevated amylase activity in serum, urine, and tumor tissue was studied electron microscopically and biochemically. Ultrastructurally, there were numerous electron-dense granules in the cytoplasm of most tumor cells. These granules were located in the apical region of tumor cells and had a single limiting membrane, associated with a clear zone just beneath it. Furthermore, circular, lamellar and annulate structures, which closely resembled those observed within zymogen granules of the salivary glands in postnatal mice and rats, were sometimes recognized within these granules. These observations suggested that these granules were identical to a postnatal or immature form of zymogen granules in salivary glands. Biochemical analysis confirmed that the amylases obtained from the tumor tissue were composed of the sialic acid-containing glycoprotein, which was not found in normal amylases. A possible histogenesis of the amylase-producing lung cancer was briefly discussed.