Fibrin film is prepared by compaction in one dimension of a fibrin clot (pH 6.3, ionic strength 0.15, fibrin concentration about 0.5%) by expulsion of fluid to reach a fibrin concentration of about 15%. Strips of film, equilibrated in the same buffer with very slowly increasing temperature, shrink in length in a narrow temperature range, as reported in 1962 by Loeb and Scheraga. The transition temperature was found to be 54 +/- 2 degrees C independently of whether the film was unligated or ligated (cross-linked) by Factor XIIIa and whether the film had previously undergone stretching with about 50% stress relaxation at a relative length of 1.23 to 1.44 and subsequent stress-free retraction. The percentage of linear shrinkage in buffer was about 32%. The transition corresponds to that observed calorimetrically by Mihalyi and Donovan in both fibrinogen and fibrin and by Medved' and Privalov in fibrinogen, localized in the D fragment. It is attributed to unfolding of structures in the D domain.