The activity of nonspecific alkaline phosphatase (E.C. 3.1.3.1) in developing teeth and bone of human fetuses and young macaque monkeys has been studied by means of histochemistry. The incubations for alkaline phosphatase were performed at pH 8.2 using naphthol-AS-MX-pnosphate as substrate and Fast Blue RR salt or Fast Red Violet LB salt as couplers. By means of pretreatment with heat (56°C), or addition of sodium metavanadate, ortho-or pyrophosphate, two alkaline phosphatases were demonstrated in the developing teeth.Prior to hard tissue formation all alkaline phosphatase activity was inhibited by the addition of vanadate, phosphate, or by pretreatment with heat. Pretreatment with heat or addition of vanadate or phosphate also inhibited alkaline phosphatase activity in the odontoblasts and in the pulpal connective tissue, whereas the activity in the subodontoblastic cell layer, stratum intermedium, outer enamel eptithelium, and the outer cells of the reduced enamel epithelium were much less affected. A week resistant activity was also noted in odontoblasts and pulpal connective tissue.