The effect of deoxyribonucleoproteid (DNP) solubilization under conditions of the ionic strength of the medium approaching the physiological one depended greatly after the treatment with a mutagen - N-nitroso-N-methylurea (NMU) on the NMU concentration. Apparently, for the change of DNA into a soluble state the critical number of groups in the DNA and protein should be modified. On the basis of the data obtained by circular dichroism and viscosimetry a conclusion was drawn that after the NMU treatment the DNP complex became soluble in the solvents with the ionic strength approaching the physiological one, chiefly as the result of labilization and dissociation of DNA-protein bonds.