A disulfide-bonded fragment with a molecular weight of about 100,000 was identified in the medium of cultured chick cranial bone and its derivation from procollagen was established by immunological criteria. The molecular weight of the fragment was reduced to 33,000 after cleavage of disulfide bonds, indicating a triple-stranded structure. The amino acid composition of the fragment lacked hydroxyproline and hydroxylysine and differed markedly from that of collagen in other respects. A similar but somewhat larger fragment was isolated after bacterial collagenase digestion of chick bone procollagen purified by chromatography on DEAE-cellulose. The characterization and comparison of these fragments further define the nature of the additional regions in procollagen and, when combined with information derived from studies of acid-extracted and dermatosparactic procollagens, support a mechanism for the conversion of procollagen to collagen which involves more than one proteolytic step.