E nhanced denaturation of type II collagen fibrils in femoral condylar cartilage in osteoarthritis (OA) has recently been quantitated immunochemically (Hollander, A.P., T.F. Heathfield, C. Webber, Y. Iwata, R. Bourne, C. Rorabeck, and A.R. Poole. 1994. J. Clin. Invest. 93:1722-1732). Using the same antibody that only reacts with denatured type II collagen, we investigated with immunoperoxidase histochemistry (results were graded for analysis) the sites of the denaturation (loss of triple helix) of this molecule in human aging (at autopsy, n= 11) and progressively degenerate (by Mankin grade [MG]) OA (at arthroplasty, n= 51) knee condylar cartilages. Up to 41 yr, most aging cartilages (3 of 4) (MG 0-4) showed very little denaturation. In most older cartilages, (4 of 7) (MG 2-4), staining was observed in the superficial and mid zones. This pattern of collagen II denaturation was also seen in all OA specimens with increased staining extending to the deep zone with increasing MG. Collagen II staining correlated directly both with MG and collagen II denaturation measured by immunoassay. Cartilage fibrillation occurred in OA cartilages with increased penetration of the staining for collagen II denaturation into the mid and deep zones and where denaturation was more pronounced by immunoassay. Thus in both aging and OA the first damage to type II collagen occurs in the superficial and upper mid zone (low MG) extending to the lower mid and deep zones with increasing degeneration (increasing MG). Initial damage is always seen around chondrocytes implicating them in the denaturation of type II collagen.