Ergot of rye is a powerful stimulus to calcification of bone. The factor responsible for this action is soluble in alcohol and ether, resists saponification and has the properties, so far as these are known, of vitamin D. Rye germ itself, unaffected by the ergot fungus Claviceps purpurea, contains a small quantity of calcifying substance which can be extracted by light petroleum. Irradiation of unground ergot grains by strong sunlight produces no increase and irradiation by the mercury-vapor lamp only a slight increase in the calcifying activity of ergot, although there is abundant ergosterol present.