An electron microscope study has been made of collagen fibrils before and after treatment with various cross-linking reagents. In general it was found that cross-linking resulted in an increase in the size of the light-staining bands (as visualized by negative staining) and it was concluded that the light bands in native collagen correspond to regions where the tropocollagen macromolecules are bonded together. Treatment of the fibrous-long-spacing form of collagen resulted in the long period of 2300 angstroms periodicity.