The interaction between high-molecular-weight DNA and a synthetic polycation (polyallylamine) in solution was studied. The formation of compact structures accompanied by substantial changes in the circular dichroism spectrum of DNA was observed in low-ionic strength solutions at close concentrations of DNA monomeric residues and polyallylamine. At lower polyallylamine concentrations, the optical anisotropy and the size of DNA molecules remained virtually unchanged. Studies of the protonation of DNA after the formation of its complexes with polyallylamine showed that the nitrogen bases of the macromolecule were protected from protonation in the compact structures.