1. Partial hydrolysates of phytic acid, containing mixtures of different polyphosphate esters of inositol, have been shown to be potent inhibitors of the calcification in vitro of ricketic rat cartilage and rat aorta; phytate itself was inert. 2. Phytic acid, as well as its hydrolysates, when injected parenterally, prevented aortic calcification in rats with vitamin D intoxication. 3. The rat appears able to dephosphorylate parenterally administered phytate. It may be necessary to reconsider the biological role of phytate.