Antigenic determinants of Mistletoe Lectin I, a toxin from Viscum album were predicted on the basis of the primary amino acid sequence of the protein. Based on the results of analysis, the peptide FPGGSTRTQARS, which corresponds to the 144-155 segment of the viscumin A-chain, was synthesized. The peptide was tested in enzyme-linked immunosorbent asssay with monoclonal antibodies against the viscumin A-chain obtained previously. The peptide reacted with antibodies with a low affinity and did not inhibit the binding of viscumin molecule to any of the antibodies. Analysis of the peptide by1H-NMR spectroscopy in aqueous solution was performed. The three-dimensional structure of the 144-155 segment in the native protein globule was shown.