This paper describes two new immunoassays for a peptide of the triple helix of type II collagen (Coll 2-1) and its nitrated form (Coll 2-1 NO(2)). In healthy subjects aged between 20 and 65 years old, Coll 2-1 and Coll 2-1 NO(2) levels in serum were in means 125.13+/-3.71 and 0.16+/-0.08 nmol/l, respectively. These levels did not significantly vary with age. However, up to 45 years of age, Coll 2-1 NO(2) levels in women were significantly higher than in men. In patients with knee osteoarthritis (OA), Coll 2-1 in serum was found to be elevated compared to healthy controls (267.45+/-26.42 nmol/l vs 126.78+/-6.61 nmol/l). Further, we have demonstrated that an increase of the urinary levels of Coll 2-1 or Coll 2-1 NO(2) over 1 year was predictive of joint space narrowing progression in OA patients. In conclusion, these preliminary results indicate that Coll 2-1 could be a predictive marker of knee OA progression.