Abstract Certain gelatins can be reversibly transformed from the disordered random state to the fibrous form with the long-range order typical of native collagen. The reconstituted collagen fibers behave normally in that they will dissolve in dilute organic acids and can then be reprecipitated in the SLS form by the addition of ATP. Gelatins of varying origin were screened for their ability to undergo the gelatin-collagen (G-C) transition. Acid or neutral extract gelatins could be carried through the G-C transition; alkali-pretreated gelatins could not. Fractionation and molecular characterization studies, particularly by ultracentrifuge techniques, showed that only high molecular weight, network structure gelatins can reform the native collagen structure. A gelatin fraction with a molecular weight near that of tropocollagen undergoes the G-C transition most readily. The existence of such gelatins implies that intratropocollagen bonds may be present and that these bonds are of nearly the same strength as the peptide chain bonds.