1. 239 blood donors were examined for the total activity of alkaline plasma phosphatase and the L-phenylalanine inhibited fraction in relation to age-, sex- and blood group dependency. In certain cases the secretor status was also examined. 2. The total activity in blood group O was significantly higher compared to blood group A 1. Men in both groups up to the age of 50 showed significantly higher total activities than women. Above this age no differences could be found between men and women. For the total population as well as for the blood group O and A 1, the values followed a log-normal distribution. 3. The L-phenylalanine inhibited fraction showed higher values in blood group O compared to blood group A 1. In comparison to the total activity no differences between age or sex could be found. Within both groups two populations were found for the L-phenylalanine inhibited fraction which was also log-normally distributed. In the individuals with the blood group O, secretors appear to have higher values of plasma alkaline phosphatase than non secretors.