The problem of the biochemical quantification of long term human wound healing was approached by measuring collagen synthesis in reincisions using specific radioimmunoassays for the wound fluid concentrations of the carboxyterminal propeptide of type I procollagen (PICP) and the aminoterminal propeptide of type III procollagen (PIIINP). First-day wound fluid PICP concentration after reincising a 3 week old scar was 25 times higher than the mean value in 20 reference standard incisions but scars older than 3 months did not show this difference. Wound fluid taken in subsequent days demonstrated that the initial acceleration of synthesis disappeared by the fourth day. When wound fluid PIIINP was assessed, high concentrations were found in reincisions of wounds for up to 5 months after the previous operation. The acceleration was also lost more slowly during the first postoperative week. The duration of a high rate of type I collagen synthesis compares well with studies in experimental wounds which show increased gain of strength if they are made not more than 6 weeks after previous surgery. The longer activity of the metabolism of type III collagen related antigens could reflect their function in the regulation of collagen fibril formation.