A technique for measuring the alkaline phosphatase activity of serum protein fractions separated by electrophoresis on cellulose acetate strips is described. Alkaline phosphatase activity in alpha 2, alpha 1, and beta globulins and in albumin is present in varying proportions in normal and pathological sera. In normal sera peak activity is in the alpha 2 globulins and beta globulins have a little more than half the alpha 2 activity. Albumin activity is often more than 10% of the total, but alpha 1 and gamma globulin activity is less than this. In bone disorders with a high serum alkaline phosphatase a large, and sometimes predominant, part of the increase is due to raised beta globulin phosphatase activity, and alpha 2 activity is also raised. Alpha 1 activity, though usually low, may be increased. In hepatobiliary disorders with a high phosphatase concentration the increase is due mainly to raised alpha 2 phosphatase activity, and there is also an increase in alpha 1 phosphatase. Beta activity is usually, but not always, low, and albumin activity may be somewhat increased in obstructive jaundice and liver metastases. There is some overlap between these two patterns of alkaline phosphatase distribution. The findings are consistent with the possibility that alkaline phosphatases in different protein fractions are derived from different tissues.