1. The utilisation of radioactive sulphur in vivo has been demonstrated both macroscopically and microscopically during the preosseous stage of bone repair.

2. The labelled mucopolysaccharide complex, chondroitin sulphuric acid, has been studied during the formation of the medullary and periosteal blastemata in the healing of a fracture.

3. The appearance and possible significance of mast cells adjacent to a fracture, and resulting from the stimulus of trauma, are discussed.

4. Cortisone has been seen to affect the formation of the periosteal cartilaginous blastema and subsequent process of endochondral ossification, with liberation of increased amounts of chondroitin sulphuric acid which was calcified rather than ossified.

1. A method is described of demonstrating in vivo the utilisation of radioactive sulphur³⁵ and of radioactive phosphorus³² during bone growth and repair.

2. The relationship between labelled chondroitin sulphuric acid and labelled phosphate complexes has been studied, the importance and significance of vascularity and the localisation of the enzyme alkaline phosphatase being noted.

3. It was found that bone growth by external accretion, both epiphysial and periosteal, was accompanied by an increased utilisation of radioactive chondroitin sulphuric acid and calcium phosphate complexes.

4. During repair in a fracture site, although there was deposition of radioactive phosphate, no preferential localisation of radioactive sulphur was observed and the possible explanations of this are discussed.