Absorbtion and fluorescence spectra of various bisbenzimidasole derivatives were studied to estimate the efficiency of their binding to DNA frow salmon testes. The effect of some radicals formed by the phenyl and piperazine rings of the fluorochromes on their interaction with a polynucleotide was determined. It was demonstrated that the Hoechst 33258 fluorescence's increase in presence of rRNA is due to the double-stranded hairpin structures in this polymer.