As the determination method of serum calcium and calcium ion which have been generally used are very complicated and indirect methods, they are very inconvenient for clinical uses. We are now working on a method which we hope will prove more .direct, easy, exact and speedy. J. Furuhata in Japan reported that he found the color reaction of Belirium using a new reagent. It is powder of purple color. It becomes red solution when it is dissolved in absolute alcohol, acetone and water, but it is not dissolved in benzene and ether. When sulfric acid of hydrochloride is added to the solution, the color change can not be seen, but in the case of added sodium hydroxide solution, the color reaction is purple and added calcium and Belirium, the color reaction is red. Using this phenomenon we found the following determination ethod.