In the microsomal fraction isolated from rat incisor odontoblasts, the influence of several inhibitors onp-nitrophenyl phosphatase activity (p-NPPase) was demonstrated at pH 10.3 and compared with the influence at pH 8.8. The inhibiting effect of EDTA on thep-NPPase activity was reversed by equimolar concentrations of Zn2+. The inhibitory effect of three diphosphonates, ethane-1-hydroxydiphosphonate (EHDP), methanediphosphonate (MDP), and dichloromethanediphosphonate (Cl2MDP) was equal to that of a chelating agent. Tetramisole and levamisole exerted an inhibitory effect that was different from the complexing type. l-Cystein, but not l-ascorbic acid, inhibitedp-NPPase activity with a maximum inhibitory effect at pH 10.5.