Psammoma bodies in meningocytic whorls were investigated by electron microscopy. In some whorls, connective tissue fibers were seen and membrane-bound vesicles were contiguous to degenerated cells. Some small vesicles, 0.1 to 0.5 micron in diameter, were outlined by plasma membrane (matrix vesicles), other larger ones, about 1 to several micron in diameter, were invested by a thick wall (matrix giant bodies). Mineralized deposits were frequent in these vesicles and occasionally large masses of mineralized connective tissue fibers (psammoma bodies) were seen. Analysis of the material in the mineralized vesicles and fibers, using an energy dispersive X-ray microanalyzer, showed that both calcium and phosphorous were evident and hydroxyapatite was substantiated using an X-ray diffractometer. Psammoma body formation in the meningocytic whorls may represent degeneration in some whorls of the central cells which contain connective tissue fibers, producing cell debris such as membrane invested vesicles. Subsequently, calcification occurs in these vesicles, and the mineralization process extends to neighboring connective tissue fibers. The calcified mass forms a psammoma body.