Studies on transglutaminases usually focus on the polymerization of protein substrates by intermolecular N(epsilon)(gamma-glutamyl)lysine bridges, without considering the possibility that the monomeric protein units, themselves, could also become crosslinked internally. Both types of crosslinks are produced in the reaction of fibrinogen with red cell transglutaminase. We isolated the transglutaminase-modified, mostly monomeric form (92-96%) of fibrinogen with a N(epsilon)(gamma-glutamyl)lysine content of approximately 1.6 moles/mole of fibrinogen. The preparation was fully clottable by thrombin, but the rates of release of fibrinopeptides and clotting times were delayed compared with control. Hybrid Aalpha.gamma type of crosslinking, the hallmark of the reaction of the transglutaminase with fibrinogen, occurred by bridging the Aalpha(408-421) chain segment of the protein to that of gamma(392-406). Rotary shadowed electron microscope images showed many monomers to be bent, and the crosslinks seemed to bind the otherwise flexible alphaC domain closer to the backbone of fibrinogen.