Fibrinolysis inhibiting factor is a peptide that mimics the N terminus of the alpha chain of fibrin. It binds to the polymerization pocket in the alpha chain more tightly than a peptide corresponding to the chain native sequence GPRV. The complex between GPRP and fibrinogen causes an increased resistance to degradation by plasmin, and the polymerization reaction is depressed by the addition of excess peptide. Fibrin assembly and the acceleration of the factor XIIIa reaction can be prevented by this homologue of the natural sequence of amino acids.