The presence of a cross-linking interaction beyond the terminal region of the molecule was established in corneal collagen. The sequence of the terminal regions of corneal collagen did not differ substantially from the sequence in the skin collagen preparations, though minor differences could not be excluded. Most of the skin preparations of insoluble collagen contain tightly attached peptidic material, which was found to be absent in the corresponding corneal preparations. Whether the high resistance of corneal collagen to the solubilizing agents is due to the presence of glycosaminoglycans remains unclear. It has been proved that they are very tightly bound to the insoluble fraction.