Abstract Seventeen peptides have been isolated from a tryptic digest of soluble elastin by ion exchange chromatography and gel filtration. Molecular weights range from approximately 1,700 to 10,500. Partial or complete amino acid sequences of the peptides were obtained and provide the first extensive data on the primary sequence of elastin. Several important findings stand out. First, the sequences obtained reveal an elastin primary structure quite distinct from collagen and possessing repeats of a tetrapeptide, Gly-Gly-Val-Pro, a pentapeptide, Pro-Gly-Val-Gly-Val, and a hexapeptide, Pro-Gly-Val-Gly-Val-Ala. Second, several of the peptides contain a partial hydroxylation of proline residues thereby definitely establishing hydroxyproline as a real constituent of the elastin chain.