Type I collagen accounts for most of the organic matrix of bones, but it is also an important constituent of soft connective tissue. Assessment of the turnover of such collagen is particularly relevant to bone metabolism. Determination of the carboxyterminal propeptide of type I procollagen (PICP) is a means of estimating the rate of type I collagen synthesis in the body. Serum concentrations of PICP have been shown to correlate with the rate of bone formation.