The staining of chicken erythrocyte nuclei with Alcian blue followed by a treatment with ethanol-NaOH, showed to be highly resistant to acid extraction (5 N HCl) due to the alkaline conversion of Alcian blue into the insoluble pigment Monastral fast blue. It is assumed that the intercalative binding of the planar Monastral fast blue molecule to DNA accounts for its competition effect toward the intercalating fluorochromes acridine organe, ethidium bromine and daunomycin.