In the present paper we report the results of a study on the base specificity and affinity of eight dyes potentially able to interact with DNA. These compounds include four triphenylmethane dyes used in histochemistry, auramine, "Hoechst 33258" and two acridines substituted with t-butyl groups. They were selected with regard to their inability to intercalate between the base pairs of helical polynucleotides due to structural limitations. Hydrodynamic studies performed with the DNA complexes of crystal violet and Hoechst 33258 confirmed our assumptions that compounds of this type bind to the outside of DNA. The main results from DNA binding studies indicate that the triphenylmethane dyes except p-fuchsin are bound with high preference to two adjacent A - T pairs while Hoechst 33258 seems to need three A - T pairs as the binding site. Model studies with synthetic polynucleotides revealed that not only a sequence of A - T pairs, but also their structural arrangement in a helix, is crucial for the high affinities observed for most of the ligands when interacting with natural DNA. Methyl green and Hoechst 33258 can be used for increasing the resolution power of cesium chloride density gradients for DNAs with different (A + T) content.