1. Histochemical studies have been made of the distribution of alkaline phosphatase, glycogen and acid mucopolysaccharides in normal growing bones (mice, rats and men) and also in forty cases of pathological bone processes (neoplastic and dystrophic).

2. The study of normal material confirmed that alkaline phosphatase is plentiful in calcification of cartilage and even more plentiful in bone formation (whether enchondral or direct).

3. It was observed that glycogen increased in the cartilage areas about to be calcified, and that it disappeared in those calcified. It seemed that osteoblasts did not always contain glycogen.

4. In the pathological material (tumours and dystrophic processes) there was great phosphatase activity in the osteogenic areas and also in the cartilage about to be calcified. Whereas glycogen was plentiful in some cases of neoplastic or reactive osteogenesis, it was absent from others.

5. In every area of normal or pathological ossification, the presence of phosphatase seems to be a rule; glycogen is often but not always present.

6. It appears that alkaline phosphatase plays an important role in the formation of the protein matrix of bone, but is not associated with the elaboration of the mucoprotein cartilage matrix.

We believe it is premature to draw any definite conclusion on the behaviour and role of the metachromatic substances in the processes of calcification and ossification.

The histochemical study of alkaline phosphatase has shown that this is a valuable method in the detection of reactionary or pathological osteogenic processes which in some cases are difficult to demonstrate with the usual histological methods.