High concentrations of zinc have been found by a histochemical method at the sites of calcification. Zinc has been detected in the developing osteons (Haversian systems) of compact bone exactly at the border line between calcified and uncalcified tissue (Fig. 1), in the cartilaginous partitions of hypertrophic cells (Fig. 3), and in the endochondral bone which has just been deposited in the metaphysis (Fig. 4).