Utilizing disodium phenylphosphate as substrate and determining the amount of phenol set free by its reaction with red B salt, Keplan and Narahara devised a sensitive micro method for the determination of serum alkaline phosphatase. We have studied the method and found it, with a few modifications, convenient and accurate. The chief advantages of this method lie in the fact that it requires serum samples of only 0.01 ml. and a reaction time of only 30 minutes, and that the whole procedure can be completed in one test tube. In order to make the method applicable to the examination of infants and small laboratory animals, we have introduced a glass capillary method for collecting cutaneous blood and separating small amounts of serum, and a micro pipet to deliver 0.01 ml. accurately. With this method, the serum alkaline phosphatase of 55 normal adult Chinese of both sexes ranged from 0.4 to 3.0 units per cc. and averaged 1.68 units with a standard deviation of ? 0.63 units and standard error of the mean of ? 0.09 units. Parallel determinations with Bodansky's method and this method were done on a series of 46 individuals, the ratio of the unitage of serum alkaline phosphatase by Bodansky's method to that by present method was found to be on the average, approximately 2. The serum alkaline phosphatase of a small series of normal rats was found to average 15.2 units per cc. A preliminary trial with rats to ascertain the effect on serum alkaline phosphatase of rachitogenic diet and vitamin D failed to give consistent results. This aspect of the work requires further studies.