the structural basis of the wide variability of the physical properties of fibrin clots and the process of assembly of the clot were investigated by electron microscopy of fibers formed under various ionic conditions. in addition, highly specific proteolytic enzymes from different snake venoms were used to remove selectively only the a (batroxobin) or the b (venzyme) fibrinopeptides from fibrinogen, in contrast to thrombin, which removes both pairs fibers produced by cleavage of only the b fibrinopeptides displayed a characteristic band pattern indistinguishable from that of fibers formed upon removal of either the a fibrinopeptides alone or of both pairs. computer modeling studies suggest that there is a unique molecular packing that gives rise to this fibrin band pattern. these findings imply that the release of either fibrinopeptide triggers similar modes of aggregation: the intermolecular binding sites can be localized to particular molecular domains. the diameters of fibers formed with each condition of enzyme, ph, salt concentration, and temperature were measured from electron micrographs. all fibers, except for those produced at both high ionic strength and ph, had about the same average diameter of 85+or-13 nm