The influence of low molecular weight components, normally present in serum, on the enzyme-catalyzed hydrolysis of inorganic pyrophosphate (PPi-ase) by serum from healthy, human individuals has been investigated in view of the known effects of divalent cations on the PPi-ase activities of enzymes in the tissues (EC 126.96.36.199., EC 188.8.131.52., EC 184.108.40.206.). Dialysis removes an apparent endogenous inhibitor and causes a slight decrease in activity; magnesium restores the activity. Calcium causes a decrease in activity at Ca:PPi concentration ratios higher than approx. 1:2. Magnesium has no influence on the activity at acid pH. At alkaline pH the effects depend on the buffer components, the pH and the Mg:PPi concentration ratio. At ratios less than 2:3 a slight increase in activity occurs; at ratios higher than 2:3 a marked decrease in activity is found. The activity-substrate (PPi) relation obeys the Michaelis-Menten kinetics only if the Mg:PPi concentration ratios are fixed. Apparent values of Km at Mg:PPi ratios of 1:2 and 1:1 were 0.167 mM and 0.095 mM, respectively. The effects of dialysis of serum and of magnesium and calcium on the PPi-ase are interpreted primarily as an effect on the state and concentration of the substrate, PPi, and less as a modification of catalytic properties of the enzyme.