The hips and knees of 170 unselected elderly adult New Zealand white rabbits were examined for the presence of chondrocalcinosis. The techniques employed were macroscopic examination, light microscopy including histochemical staining, polarizing microscopy, autoradiography after the intra-articular injection of ³⁵S, ³H-proline, and ³H-glycine, and x-ray diffraction. All joints were examined macroscopically and microscopically. The joints showing macroscopic evidence of chondrocalcinosis were studied in detail. Autoradiography was employed in three animals which showed gross evidence of chondrocalcinosis. Serum calcium determinations were performed on some rabbits.

The incidence of abnormal mineralization was 12 per cent by macroscopic examination and 16 per cent by both macroscopic and microscopic examination—a low figure since only ten randomly selected sections from the femoral heads of the animals not showing gross evidence of calcification were examined.

Microscopically the deposits were sharply delineated from the surrounding normal cartilage and were located primarily in the deeper and mid-portions above the tidemark. In general the surrounding cartilage appeared normal but in some instances where the tissue was well preserved and fixed there was evidence of death and loss of chondrocytes adjacent to a deposit without mineral deposition in either the matrix or the cells. Increase in basophilia and in the uptake of ³⁵S, ³H-proline, and ³H-glycine was evident in the cells and matrix surrounding the lesions, suggesting an attempt at healing.

By x-ray diffraction all of the mineral was shown to be hydroxyapatite. The calcium-phosphorus ratio in ashed samples was 1.66 corresponding exactly to the stoichiometry of hydroxyapatite. The serum calcium levels of twenty rabbits were normal.

The literature is reviewed and the possible mechanisms involved in the production of these deposits are discussed.