Proteoglycans from bovine nasal septa or the Swarm rat chondrosarcoma, as potassium salts, effectively inhibit the precipitation of tricalcium phosphate in vitro at pH 7.8. The same preparations, and many other similar preparations, however, do not site bind calcium, as assessed with a calcium ion specific electrode. However, after treatment of aggregate preparations of proteoglycans with EDTA, the preparations can site bind calcium. The amount thus bound is approximately equal to one-half the sum of the equivalents of the ester sulfate and the uronic acid carboxyl groups in the preparations. This latter observation suggested the possibility that the supposed potassium salts of the proteoglycans had, in the course of preparation, acquired calcium and held onto it strongly. In checking this possibility, using neutron activation analysis, it was found that some of the preparations do contain small amounts of calcium but these amounts are insufficient to saturate the binding sites potentially available to this end. In view of the above observations, it is suggested that the proteoglycans inhibit the formation of calcium phosphate precipitates in vitro not because the calcium is site bound but because the calcium ions are territorially bound.