A disulfide-cross-linked collagen has been extracted with neutral salt solutions from organ cultures of embryonic chick sternal cartilage. This collagen, which we term pM collagen, is presumed to be the native extracellular precursor molecule to disulfide-cross-linked collagen fragments recently described. Cleavage of pM collagen under native conditions with pepsin gives rise to the collagen fragments M1 and M2, which had also been isolated from pepsin extracts of chick hyaline cartilage [K. von der Mark, M. van Menxel & H. Wiedemann (1982) Eur. J. Biochem. 124, 57-62]. Native pM collagen was purified by DEAE-cellulose chromatography and agarose gel filtration. On agarose and following polyacrylamide gel electrophoresis, the unreduced molecule migrates with an apparent Mr of 300 000. Reduction of disulfide bridges produces two subunits with Mr 80 000 (pMa) and 60 000 (pMb) when compared with collagen standards. Cyanogen bromide cleavage of pMa and pMb, excised from dodecyl sulfate gels, resulted in different peptide maps, indicating that both components are genetically distinct polypeptide chains. The occasional appearance of the unreduced pM collagen as a doublet band on dodecyl sulfate gels and the observation that pMa and pMb occur in non-stoichiometric ratios suggests that pMa and pMb form separate native molecules, although their incorporation into a single pM molecule cannot be excluded. Native pM collagen was completely digested with bacterial collagenase, and contained hydroxyproline and proline in a ratio of 1.15:1, indicating the absence of significant non-collagenous domains. Thus it represents, despite several pepsinlabile sites, more likely a largely triplehelical, processed form of collagen rather than a procollagen-like molecule containing globular domains. Processing of pM collagen to M1 and M2 fragments or other intermediate forms was not observed in cartilage organ culture or in chondrocyte cell cultures within 18 h.