1. Experimental fracture callus in rats contains mast cells as a normal morphological element.

2. The mast cell count undergoes peculiar variations in the normal course of events in experimentally delayed or accelerated bone repair.

3. A hypothesis is presented in which the tissue mast cell granules are regarded as calcium transporters in the mineral phase of callus formation, a process probably corresponding to Selye's concept of "mastocalciphylaxis" and "mastocalcergy."