AbstractCollagen exhibits a low angle x‐ray diffraction pattern corresponding to a spacing of about 650 A. On denaturation (shrinking) this pattern disappears and does not return, regardless of how the material may be treated subsequently. It has been found that, when collagen tendons are stretched taut in a clamp, immersed in water, and then subjected to temperatures above the shrink temperature, the intensity of the low angle pattern diminishes appreciably. This has been interpreted as a decrease in the concentration of the structure which gives rise to the pattern and was assumed to be due to a melting of the crystalline structure of collagen. At constant temperature (above the shrink temperature), the low angle pattern continually decreases in intensity with increased shrinkage until it disappears completely. The extent to which the sample must be shrunk before the pattern disappears is dependent upon the temperature and increases with decreasing temperature.On analyzing the data quantitatively, using the theories of crystallization for linear polymers developed by Flory, it was possible to calculate a heat and entropy of shrinkage. The values obtained for these quantities indicate that the fusion is a stepwise process involving very few amino acid residues in each step (perhaps only one or two residues).Experiments were carried out on collagen shrunk in water and in formamide, as well as on formaldehyde tanned collagen shrunk in water. It was found that the heat and entropy of shrinkage apparently decrease with increasing tendency toward swelling. Thus the heat of shrinkage for formaldehyde tanned collagen is greater than for untanned, and the heat of shrinkage for native collagen is greater in water than in formamid