To elucidate the origin of collagen hyperfibrils, we have ultrastructurally investigated the development of collagen fibrils after in situ treatment of liver tissue sections by different proteolytic enzymes. After treatment, collagen fibrils were largely disorganized within the same fibril bundle. A wide range in diameters was observed. Hyperfibrils, which correspond to those fibrils with the largest diameters and having a flower-like section, were present. On the basis of these results, we can suggest that hyperfibrils reflect breakdown states of collagen fibrils.