The tissue distribution and developmental appearance of alkaline phosphatase and Gla proteins, relative to mineralization were examined. Undecalcified sections of tooth germs, calvaria and alveolar bone of the rat were immunostained for Gla proteins, (indirect immunofluorescence and avidin-biotin-peroxidase-complex) and then stained for alkaline phosphatase activity. In the growth regions of tooth germs, Gla protein was observed in young odontoblasts and in early predentin, at a stage prior to formation of the first mineralized dentin. Similar results were obtained for bone: young osteoblasts as well as osteoid were immunopositive for Gla protein prior to formation of the first mineralized bone. Histochemical staining revealed that differentiating odontoblasts and osteoblasts exhibited alkaline phosphatase activity at stages prior to appearance of Gla proteins and that cells adjacent to odontoblasts and osteoblasts (not directly involved in the formation of predentin/dentin and osteoid/bone) stained for alkaline phosphatase, but not for Gla proteins. We conclude, that in these bone- and dentin-forming cells, alkaline phosphatase activity is expressed before the appearance of Gla proteins, but that both appear before the onset of mineralization. We also conclude that Gla protein is a more specific marker for bone and dentin formation than histochemical alkaline phosphatase activity, since only the cells directly involved in these processes were immunostained for Gla protein.