Fibril formation of collagen from porcine skin was studied at temperatures of 28-39 degrees C. Collagen fibrils obtained in this temperature range have the different degree of order. An optimal temperature for the formation of collagen fibrils is found to be 36.5 degrees C; the structure of fibrils formed at this temperature is more homogeneous than that formed at 34.5 degrees C and 38.5 degrees C. As indicated by electron microscopy data, fibrils with a minimal diameter are formed at physiological values of pH, ionic strength, and temperature. The greater diameter of fibrils formed at 34.5 degrees C is due to a high level of hydration of collagen molecules. Fibril diameter is larger at 38.5 degrees C owing to the cooperative unfolding of the triple helix and a weakening of the binding of collagen molecules.