Inorganic pyrophosphatase [EC 3.6.1.1] was purified from porcine brain to an electrophoretically homogeneous state. The molecular weight of the enzyme was estimated to be 62,000 by gel filtration and that of the subunit to be 33,000 by gel electrophoresis in the presence of sodium dodecyl sulfate, suggesting that the enzyme consists of two identical subunits. The stability of the purified enzyme was dependent on its protein concentration. The enzyme was stable above 50 micrograms/ml at 20 degrees C, but it was gradually inactivated below this concentration, even at 0 degree C unless other proteins such as bovine serum albumin, calmodulin, etc. were present. Those added proteins not only protected the enzyme from inactivation, but also completely reactivated the enzyme after it had been once inactivated. The enzyme catalyzed the hydrolysis of inorganic pyrophosphate but not that of other phosphate esters. Only Mg2+ was required as an activating cation, and other divalent cations inhibited the activity to some degree. The addition of sulfhydryl reagents prevented the inhibition of activity by divalent cations.