Lysyl oxidase activity was found in the isthmus (the membrane-forming region) of the hen's oviduct in a copper-rich region proximal to the shell gland. Desmosine and isodesmosine, cross-linking compounds associated with mature elastin, were found in hydrolysates of the shell membrane, confirming the necessity for lysyl oxidase in its biosynthesis. Shell membranes from hens fed a copper-deficient diet or a diet supplemented with beta-aminopropionitrile had a reduced content of desmosine and isodesmosine.