Abstract. The interaction of DNA molecule with new synthetic compounds, piperazine derivatives of benzo[4,5]imidazo[1,2-a]phthalazine, was investigated by the spectral, hydrodynamic and optical methods. The thermodynamic parameters of the interaction and the stoichiometry of the complexes were determined by spectrophotometric titration. A mode of binding and structure of the complexes were determined by analyzing the changes in the intrinsic viscosity and the optical anisotropy of the macromolecule upon complexation. It is shown that an increase in the intrinsic viscosity and the optical anisotropy with a small content of the ligand in the complex is caused by the increase in the thermodynamic rigidity of macromolecules upon the formation of the complex. Increase of the contour length of the macromolecule in the complex does not occur, that indicates nonintercalative mode of binding the piperazine derivatives of benzoimidazophthalazine with DNA.