The changes in absorption spectra in the visible region observed on adding different naturally occurring and synthetic DNA duplexes to solutions of 4,5-dibromo-2,7-di-(acetatomercuri)-fluorescein indicate that the mercurial reacts with polynucleotides of this type. The reaction is reversible as proved by adding excess of KCN which restores the original spectra of the free dye. The interaction is characterised also by quenching of the fluorescence of the dye and the induction of optical activity in it. The extent of these spectral effects depends strongly on the (A+T) content of the complexed DNA and decreased in the order: poly [d(A-T)], Clostridium perfringens DAN, Escherichia coli DNA, Micrococcus luteus DNA and poly(dC). From equilibrium-dialysis experiments the same order in affinity is obtained when these poly-nucleotides are at equilibrium with the same concentration of 4,5-dibromo-2,7-di-(acetatomercuri)-fluorescein. From the changes produced by different mercurials in the ORD spectra and viscosity of a DNA solution it has been concluded that 4,5-dibromo-2,7-di(acetatomercuri)-fluorescein does not cause any drastic alteration of the secondary structure of DNA.