The standard model for the ascending hierarchical structure of tendon collagen fibrils starts with collagen triple helices, microfibrils, subfibrils, and ends with fibrils. This model, developed primarily on the basis of X-ray diffraction results, is necessarily vague about the cross-sectional organization of fibrils and has lead to the widespread assumption of laterally homogeneous close-packing. This assumption is inconsistent with our data. Using atomic force microscopy and micro-manipulation, we observe collagen fibrils from tendons behave mechanically as tubes. We conclude that the collagen fibril is an inhomogeneous structure composed of a relatively hard shell and a softer, less dense core. To get further insight into the molecular organisation of collagen fibrils we performed pulling experiments using a small-cantilever AFM.