The significance of factor XIII (fibrin stabilizing factor) in hemostasis and wound healing is well known. The significance of fibrincrosslinking has, however, not been unequivocally demonstrated. As an additional hypothesis we postulated that factor XIII could bind fibrin to collagen by transamidination. To test this hypothesis:1) Normal plasma or F XIII deficient plasma, collagen or buffer, Ca++ and thrombin were incubated at 37°. The clots were washed and dissolved in SDS, urea and

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... hanol. Electrophoresis on 7% SDS-polyacrylamide showed disappearance of the γ-dimer in the clot of normal plasma in presence of collagen. N-acetylation of the collagen abolished the disappearance of the γ-dimer.2) To normal plasma, F XIII free plasma or normal plasma with EDTA, 125I-fibrinogen was added. The plasma-collagen mixtures were incubated with thrombin and Ca++, without Ca++ for the normal EDTA-plasma. The clots were solubilized as described. After extensive washing the radioactivity retained on the collagen was measured. The collagen from F XIII free clots had 23-47% and that from normal EDTA-plasma 23-46% of the radioactivity of the collagen from normal recalcified plasma clots.These results are interpreted as demonstration of binding of fibrin to collagen by F XIII catalyzed transamidination. The importance of this binding for normal hemostasis and wound healing is emphasized.