Objective. To determine whether oxidative damage to cartilage proteins can be detected in aging and osteoarthritic (OA) cartilage, and to correlate the results with the local production of interleukin-1β (IL-lβ) and the responsiveness of isolated chondrocytes to stimulation with insulin-like growth factor 1 (IGF-1). Methods. The presence of nitrotyrosine was used as a measure of oxidative damage. Histologic sections of knee articular cartilage, obtained from young adult and old adult cynomolgus monkeys, which develop age-related, naturally occurring OA, were evaluated. Each cartilage section was graded histologically on a scale of 0-7 for the presence of OA-like changes, and serial sections were immunostained using antibodies to nitrotyrosine and IL-1β. Chondrocytes isolated and cultured from cartilage adjacent to the sections used for immunostaining were tested for their response to IGF-1 stimulation by measuring sulfate incorporation in alginate cultures. For comparison with the monkey tissues, cartilage sections from human tissue donors and from tissue removed at the time of OA-related joint replacement surgery were also immunostained for nitrotyrosine and IL-1β. Results. The presence of nitrotyrosine was associated with aging and with the development of OA in cartilage samples from both monkeys and humans. All sections that were highly positive for IL-1β also showed staining for nitrotyrosine. However, in a few sections from older adult monkeys and humans, nitrotyrosine was present but IL-1β was absent, suggesting that some age-related oxidative damage is independent of IL-1β. In chondrocytes that were isolated from monkey cartilage positive for nitrotyrosine or IL-1β, the response to stimulation with IGF-1 was significantly reduced. In some samples from older adult monkeys, IGF-1 resistance was seen in cells isolated from tissue that did not stain for nitrotyrosine or IL-1β. Conclusion. Oxidative damage due to the concomitant overproduction of nitric oxide and other reactive oxygen species is present in both aging and OA cartilage. This damage can contribute to the resistance of chondrocytes to IGF-1 stimulation, but it is unlikely to be the sole cause of IGF-1 resistance in these chondrocytes.