Tropoelastin a was isolated from lathyritic chick aortae by using severe denaturing conditions for the initial extraction. The amino acid composition of this new species of tropoelastin is elastin-like in its high proportion of proline, glycine, alanine and valine. However, it differs from authentic tropoelastin b in containing a higher percentage of polar amino acids and cysteine residues. In addition, the amount of proline hydroxylation is 3 times higher than that found in chick tropoelastin b.