1. Elastins purified by various methods from the ligamentum nuchae and the lungs of cattle of various ages were analysed for amino acid compositions and lysine-derived cross-links. 2. In fully mature elastins from adults the main cross-links were desmosine, isodesmosine and lysinonorleucine and the so-called aldol-condensation product. Trace amounts of merodesmosine were also found. 3. During the normal maturation of elastins the amounts of desmosine, isodesmosine and lysinonorleucine increased, whereas the aldol-condensation product and intact lysine residues decreased and merodesmosine remained the same. 4. Elastins from young animals contained significant amounts of dehydromerodesmosine whereas elastins from adults contained virtually nil. Evidence is presented which suggests that the biosynthetic pathway of desmosine and isodesmosine proceeds via the aldol-condensation product and dehydromerodesmosine.