Aminopropeptides are triple-standard structures with a molecular weight of about 40,000-45,000 located at the amino end of procollagens. They are released during the conversion of procollagen into collagen by specific proteases cleaving a single Pro-Gln peptide bond. The individual peptide chains usually consist of a compact noncollagenous domain stabilized by intrachain disulfide bridges and a collagenous domain folded into a triple helix. Chemical and immunologic analyses of different types of procollagen have demonstrated certain sequence homologies but also distinct structural differences between their aminopropeptides. These peptides may be involved in the control of collagen synthesis and fibril formation. Some inherited disorders (dermatosparaxis, Ehlers-Danlos syndrome Type VII) are characterized by an incomplete release of the aminopropeptide in situ. Immunologic reagents have been developed which also allow the study of aminopropeptide metabolism in a variety of acquired connective tissue diseases.