A case of gastric adenocarcinoma with argyrophilic property and psammomatous calcification is reported. Histologically, the psammoma bodies are found most frequently within the glandular lumina. Electron microscopy, however, reveals that calcium first appears within the cytoplasm of tumor cells. Electron probe x-ray microanalysis demonstrates calcium and phosphorus in granular or floccular osmiophilic deposits found in intracytoplasmic electronlucent zones of tumor cells. By x-ray diffractometry, the calcium component is presumed to be hydroxyappatite (Ca5(PO4)3.(OH). The findings strongly support the view of the intracytoplasmic origin of psammomatous calcification. The tumor yielded a parathyroid hormone (PTH)-like substance, and a possible relationship between this substance and psammomatous calcification is spectulated.