Summary
1. The behaviour of the iodine label during coagulation was investigated in 8 species
using 125I-fibrinogen.
2. Guinea pig, human, monkey, ox and rabbit fibrinogen preserved the label quantitatively
when converted into fibrin, but goat and sheep fibrinogen lost 5.6% and canine fibrinogen
lost 11.1 % of its radioactivity.
3. Using a constant iodination technique, the fraction of radioactivity released during
clotting proved to be fairly reproducible. The origin and quantity of thrombin used
had no influence on the results.
4. The released activity was tentatively identified in the clot liquor of sheep as
125I-fibrinopeptide B. Based on this observation, a theory is presented which satisfactorily
explains the experimental findings by the variations in the occurrence of non-esterified
tyrosine residues in the fibrinopeptides of different species.