Enzyme preparations obtained from bacterial plaque, whole saliva and carious dentine were studied with respect to their ability to hydrolyze various phosphates. The substrates used were selected to represent the following biologically important phosphates: hydroxyalkyl phosphates, alkyl phosphates, enol phosphates, phosphoramidates, dialkyl phosphates, alkyl pyrophosphates, P1P2-dialkyl pyrophosphates, and alkyl triphosphates. The enzyme preparations exhibited a versatile reactivity pattern because all types of phosphates were hydrolyzed at a fairly high rate. Product inhibition (by phosphate, tested at 0.05 × 10––3 –– 1.0 × 10––3 M) was seen to be a feature of the enzymic hydrolysis of p-nitrophenyl phosphate. 1 mM MgCl2 slightly activated most of the catalyses studied. Certain quaternary ammonium compounds were seen to activate the hydrolysis of p-nitrophenyl phosphate (e. g. tetramethylammonium iodide by approximately 5 % and those with longer alkyl chains, e. g. benzethonium chloride, domiphen bromide, dequalinium chloride and cetylpyridinium chloride, by appro