The physicochemical mechanism of aggregation of fibrinogen has been investigated in the presence and absence of fibrin-stabilising factor (factor XIIIa). Data from elastic and inelastic light-scattering and viscometry show that molecules of fibrinogen undergo a spontaneous modification of their carboxyl terminals and bind 'end to end' into flexible polymer chains. On attaining a critical length, the single-filament polymers twist into a coil and aggregate to form branched molecules in which the segments are packed sufficiently densely to resemble strongly hydrated globular particles. The formation, under the influence of factor XIIIa, of epsilon/gamma-glutamyl-lysine covalent bonds produces only insignificant changes in the spatial organisation of the fibrinogen aggregates. Covalent dimerisation of the gamma-chains restricts the structural flexibility of the polymers, but linking of the alpha-chains provides progressive compaction of the structure with increase in molecular weight. Electrophoresis of reconstituted samples shows that the coil-shaped chains of fibrinogen oligomers prevent the complete enzymatic linking of the gamma-chains. The results of this work suggest that the accelerated assembly of multimolecular aggregates, seen in the presence of factor XIIIa, may be explained by the stabilisation of intermediate complexes of fibrinogen, which makes the spontaneous transition from a stable native state to the activated state irreversible.