We demonstrated that polyamines, such as spermine and spermidine, can enhance the pyrophosphatase (PPase) activity of alkaline phosphatase (ALP). Bisphosphonates such as disodium-1-hydroxy-1-aminopropylidine-1,1-diphosphonate (APD) and ethane-1-hydroxy-1,1'-diphosphonate (HEDP) inhibited ALP phosphate ester hydrolysis activity more than PPase activity at the same concentrations. This indicated that PPase activity of ALP was available in the presence of pyrophosphate analogues and possibly organic pyrophosphates as well. Vanadate and cadmium inhibited ALP and PPase activity more than ALP phosphate ester hydrolysis activity at the same concentrations. Calcium inhibited ALP PPase activity, though it did not inhibit ALP phosphate ester hydrolysis activity. At high concentrations, ascorbic acid slightly inhibited ALP PPase activity, though it did not inhibit ALP phosphate ester hydrolysis activity. ALP PPase activity appeared to have ubiquitous intracellular existence, broad substrate specificity and extensive interaction with calcium, vanadium and polyamines-substances which are important for cell metabolism and cell growth. These findings suggested that intracellular ALP modulated cell metabolism and cell growth by its PPase activity.