Using the idea of "proline brackets" we have found four sites in fibrin amino acid sequence, and appropriate peptides were synthesized: γ69NPDESSKPN77, Bβ228QPDSSVKPY236, Bβ455RPFFPQ460 and Aα195LPSRDRQHLPL205. Turbidity and electron-microscopy analyses have demonstrated that synthetic peptide Aα195-205 specifically inhibited the stage of fibrin protofibril formation and peptide γ69-77 - the stage offibrin protofibril lateral association. The data obtained testify that there are the sites involved in these processes in the appropriate amino acid sequences of fibrin molecule.