Peptides Gly-Pro-Arg-Pro and Gly-His-Arg-Pro (fibrin alpha- and beta-chain NH2-terminal analogs, respectively) are studied for their effect on fibrinogen (F) and fibrin NH2-terminal disulphide knot (N-DSK) specific binding. Both peptides are found to inhibit the formation of soluble and insoluble F-N-DSK-complexes through inhibition of the interdomain D-E-binding. Gly-Pro-Arg-Pro is much more potent inhibitor than Gly-His-Arg-Pro. Lowering the insoluble F-N-DSK-copolymer quantity by concentration-dependent way these peptides do not change its composition described by the formula [F(N-DSK)2]n. Invariability of fibrinogen and N-DSK copolymer structure is asserted. In this structure neighbouring fibrinogen molecules are bound by two N-DSK molecules via the D1-E1 and D2-E2 binding sites.