AbstractAdult and larval lamprey cartilage, normally unmineralized, was cultured in a medium meta‐stable for hydroxyapatite for periods of up to 12 days at 20, 30 or 37°C. Histochemical analysis revealed a temperature‐dependent increase over time in calcium phosphate incorporation into the extracellular matrix (ECM) of adult cartilage concomitant with the incorporation of calcium as detected by gravimetric and radioisotopic methods. Ultrastructural analysis revealed the presence of ECM dense bodies (40 nm average) containing crystalline material which increased in number with increased mineral incorporation but were absent from control cartilage. Additionally, electron‐dense particles (15–20 nm) found in close association with the ECM fibrils in some regions of mineralized cartilage may represent sites of amorphous calcium phosphate. Larval cartilage incorporated much less calcium, less uniformly over time than did adult cartilage. Histochemical and ultrastructural analysis revealed that calcium had accumulated in the chondrocytes instead of the ECM and this proved detrimental, leading to the death of the cells.The discovery that adult lamprey cartilage calcifiesin vitrounder appropriate conditions, suggests that petromyzonids, or their direct agnathan ancestors, may have possessed mineralized skeletons and that this ability is ‘repressed’ in extant lampreys owing primarily to the environment