A high correlation exists between the activities of inorganic pyrophosphatase (E.C. 220.127.116.11) and alkaline orthophosphatase (E.C. 18.104.22.168) both in native serum and in electrophoretically isolated serum protein fractions. The ratio between activities depends on the assay conditions. Over a long period, the ratio of activities in patients presenting elevated orthophosphatase activity is constant. Sera from patients with diseases of hepatobiliary organs and of the bones containing increased orthophosphatase activity do not differ significantly in their pyrophosphatase activity if urea and l-phenylalanine are added to the incubation mixture.The Michaelis constant is higher for sera related to osseous cases than for hepatobiliary cases (inorganic pyrophosphate: Km 0.334 and 0.213 mmole/1, respectively). The effect of magnesium on pyrophosphatase activity depends on pH, buffer type and concentration of inorganic pyrophosphate during incubation, but the source of the increase in orthophosphatase activity is not disclosed by varying the magnesium concentration in the assay of pyrophosphatase. Serums display a decreasing orthophosphatase activity with increasing concentrations of EDTA, but the inhibition is completely overcome by adding magnesium in excess. This is not the case with serum pyrophosphatase activity which is strongly inhibited at low concentrations of EDTA and not reactivated by magnesium.