The interaction of ethidium bromide with single-stranded synthetic and natural polynucleotides at high temperatures (t = 70 degrees C) and low pH values (pH 3.0) was studied. The isotherms of adsorption of ethidium bromide on single-stranded DNA were obtained. Two modes of binding of single-stranded DNA, strong and weak, were revealed. The values of the corresponding constants of interaction of this ligand and the number of bases per one binding site were determined.