Phosphate mixtures added to serum in vitro lead to a fall in Ca++ conc., owing to formation of a colloidal complex of Ca and phosphate. Both the rate of fall and the level of equilibrium are dependent upon the conc. of phosphate. Similar mixtures administered intraven. or orally to dogs produce a similar reaction in the plasma. Colloidal Ca phosphate when formed in the blood in vivo or when administered intraven. is quickly removed from the circulation. Following administration of phosphate to the animal, or addition of phosphate to serum in vitro the product Ca++ X total inorganic phosphate in plasma or serum (both expressed in millimols per liter) approaches a constant value of 3.0 at equilibrium. When blood is withdrawn from the animal before equilibrium is reached the reaction continues to equilibrium in vitro. The product Ca++ X P > 3.0 represents the conditions for the formation of colloidal Ca phosphate in plasma.