Mast cells in the periosteal callus of healing experimental fractures in rats were shown to incorporate35S‐Iabelled substances. During the first two weeks after fracture,i. e.the organic phase of repair, the uptake of label reached a maximum. As mineralization of the fractures proceeded, the labelling became less intense and the total number of mast cells decreased. It is discussed whether the mucopolysaccharides of the mast cells undergo metabolic changes in the process of repai