A neutral proteinase, distinct from collagenase, was isolated by gel-filtration from medium that had been conditioned by the culture of explanted porcine gingiva. This enzyme was shown to cleave the alpha 1 (I) chain carboxyterminal telopeptide in native collagen proximal to (i.e. nearer to the helix than) the lysyl residue at position 17 which is known to be important in the formation of covalent intermolecular cross-links. We refer to this activity as 'telopeptidase'. The enzyme had an apparent Mr of 70,000, as determined by gel-filtration. It was inhibited by ethylene diamine tetraacetic acid but not by N-ethyl-maleimide nor by phenylmethylsulphonyl fluoride. It is therefore probably a metalloproteinase. The pH optimum for this activity was 7.0-7.5. Incubation of the enzyme with fibrillar (acid-insoluble) calf-skin collagen resulted in solubilization of collagen in which shortening of the carboxy-terminal telopeptides could be demonstrated. It is suggested that the telopeptidase, acting within a region of the Type I collagen molecule which is known to be essential for the stability of collagen fibrils, could potentially play an important role in collagen degradation in vivo.