In order to clarify the effective regions that are substantially involved in the gelling process of plasma glycoprotein fibrinogen, we examined the aggregating properties of plasmin-treated fibrinogen, fragment-X. Two types of fragment-X were prepared by the digestion at 6°C and 37°C (fragment-XY and -XN, respectively). αC regions were cleaved thoroughly in both samples, but the amount of cleavage of BβN region differed between them (higher in the fragment-XN). Thrombin- and reptilase-catalyzed fibrin polymerizations were studied for both of the samples. Although the B-knob:b-hole interaction does neither work in the reptilase-catalyzed fibrin polymerization nor the αC-αC interaction is present, network formation proceeded in the fragment-XY with delayed polymerization. In the fragment-XN, protofibril formation occurred, but lateral aggregation did not. Mixing effect of intact fibrinogen showed that BβN region might play an important role in the lateral aggregation process cooperatively with αC-domain. .