We report here the normal range of serum alkaline phophatase activity as measured by the method proposed by Hausamen et al. [Clin. Chim. Acta 15, 241 (1967)] with a much larger sample size than used in previous investigations. In the statistical analysis the sample population is subdivided by sex and age, two variables which are known to influence enzyme activity. No statistically significant influence of blood type on enzyme activity was observed. The normal range of enzyme activity is reported in percentiles.