The age-dependent changes in reducible collagen cross-links, Schiff bases and their precursors were followed for human bone and articular cartilage throughout the whole life span from 3 to 89 years of age. With aging, the reduced Schiff bases dihydro-ylysinonorleucine remarkably decreased, whereas hexitollysine peaks increased significantly in both collagens. Dihydroxynorleucine, the precursor of the cross-link, was found to increase slightly with age. But the changes in hydroxylysinonorleucine and lysinonorleucine were comparatively small. These changes were discussed in relation to the increased insolubility observed in both collagens at higher age.