We describe a family with an apparently autosomal-dominant trait that caused extremely high circulating concentrations of the carboxyl-terminal propeptide of type I procollagen (PICP). All family members examined had normal values for other biochemical markers of bone formation and degradation and no related clinical abnormalities. Furthermore, their serum concentrations of the amino-terminal propeptide of type I procollagen (PINP) were normal. Although PINP and PICP are released from the same precursor molecule, PINP is cleared from the circulation via the scavenger receptor in liver endothelial cells, whereas PICP is cleared via the mannose receptor of these cells. We thus hypothesize that the clearance of circulating PICP is compromised in the affected subjects of this family, the result of either a defective mannose receptor function or an abnormal molecular structure of their PICP.