Precipitation (or gelation) of collagen from cold neutral salt solution induced by warming was shown to be reversible on subsequent cooling. The degree of reversibility of heat precipitation rapidly diminished with time of incubation at 37°C. For calf skin collagen (acetic acid-extracted) and guinea pig skin collagen (crude NaCl extract) in neutral salt solutions (Γ/2 = 0.45) roughly 90 per cent of newly formed gel redissolved on cooling at 2°C.; less than 20 per cent redissolved on cooling gels previously maintained at 37°C. for 24 hours. At physiologic ionic strength the same preparations exhibited much more rapid development of irreversible precipitation, but the same time dependence was clearly evident. Highly purified collagen from crude saline extracts of guinea pig skin exhibited the same phenomenon although the quantitative aspects were somewhat different.

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