Fibrinogen Aarhus is an abnormal fibrinogen for which the clotting time with thrombin is greatly prolonged both in plasma and in the isolated fibrinogen. The whole blood clotting time is only slightly prolonged. The patient with this fibrinogen has no bleeding tendency. In this report we have investigated fibrinogen Aarhus in two alternative, thrombin independent polymerization and gelation pathways. These pathways are the factor XIII dependent oligomerization and gelation of fibrinogen, and heteropolymer (fibrinogen-fibronectin) formation which also is catalysed by factor XIII. Both of these reactions are qualitatively the same in fibrinogen Aarhus as in normal fibrinogen, but the rate of oligomerization is somewhat slower in fibrinogen Aarhus. This may depend on impaired association between factor XIII and fibrinogen Aarhus.