Cytochemical mechanisms of chromatin fluorescence and staining reactions by thionin solutions

After staining with thionin at low concentration (less than 10(-5) M), the masses of condensed chromatin in chicken erythrocyte nuclei show a red fluorescence under green excitation, which is abolished when they become stained by using concentrations higher than 10(-4) M. The possibility that intercalative binding of a hydrophobic component from the dye solution into DNA accounts for this fluorescence reaction in chromatin is briefly discussed.