Staining polarization optical techniques showed differences in the structural organization of DNA of chromatin in interphase nuclei and in mitotic chromosomes. The DNA was non-birefringent in intact interphase cell nuclei, but birefringent in chromosomes and in isolated nuclei incubated in a physiological electrolyte solution. The birefringence of DNA appears to be related to an unfolding of DNA filaments induced by free cations and to the oriented binding of dye molecules to DNA phosphates. We propose that the actual concentration of free cations inside the living cell nuclei is regulated by a dynamic interaction between nuclear proteins and ions.