The Ca and inorganic P concns. were elevated and the phosphatase decreased in the serum of dogs and rabbits fed toxic doses of irradiated ergosterol (I.E.). Distension of the gall-bladder with bile was noted in practically every rabbit subjected to toxic doses of I.E. Sub-toxic doses of I.E. elevated the serum phosphatase of some dogs fed a bread and corn-meal diet, but was without effect when the ration consisted of cooked meat with added bone-meal. An inverse relationship was found between serum Ca, inorganic P, and phosphatase of dogs and rabbits influenced by I.E. Dogs fed a high protein (beef heart) diet had low serum phosphatase; changing to a high carbohydrate (bread and cooked corn-meal) diet invariably raised the phosphatase activity. An inverse relation appears to exist between the conc. of acid-soluble organic P of whole blood and the activity of serum phosphatase.