Lower incisors of 3 week old mice were decalcified for 24 hours in buffered 10% EDTA at 4[degree]C. Frozen sections were fixed for 1 hour in cold 10% formol-Ca, then incubated in naphthol AS-TR phosphate with hexazonium pararosanilin as coupler. Ameloblasts of the epithelial loop showed marked dye deposits throughout their cytoplasm, but in the zone of matrix formation enzyme activity was supranuclear. As ameloblasts matured and became atrophic, the area and intensity of enzyme activity gradually decreased. Odontoblasts showed moderate dye deposits, the pulp was weakly active. Enamel and dentin remained blank. Acid phosphatase activity was seen in ameloblasts in the same areas in which lysosome like structures occur. The findings do not permit a definitive conclusion regarding the possible role of acid phosphatase in amelogenesis.