Observations on the chromatin staining and fluorescence by safranine O

After treatment with safranine O at low concentrations (10(-6), 10(-5) M), the chromatin of chicken erythrocytes shows an intense red fluorescence, which is reduced or practically abolished when nuclei become stained by using the dye at concentrations higher than 10(-4) M. Both the fluorescence and staining reactions are dependent on the DNA content of chromatin. Safranine fluorescence of nuclei is not reduced in presence of high salt concentration, while the previous treatment with thiazine dyes diminishes the fluorescence intensity. The possibility that chromatin fluorescence depends on the intercalative binding of safranine is suggested.