A reproducible Romanowsky-Giemsa staining (RGS) can be carried out with standardized staining solutions containing the two dyes azure B (AB) and eosin Y (EY). After staining, cell nuclei have a purple coloration generated by DNA-AB-EY complexes. The microspectra of cell nuclei have a sharp and intense absorption band at 18,100 cm-1 (552 nm), the so called Romanowsky band (RB), which is due to the EY chromophore of the dye complexes. Other absorption bands can be assigned to the DNA-bound AB cations. Artificial DNA-AB-EY complexes can be prepared outside the cell by subsequent staining of DNA with AB and EY. In the first step of our staining experiments we prepared thin films of blue DNA-AB complexes on microslides with 1:1 composition: each anionic phosphodiester residue of the nucleic acid was occupied by one AB cation. Microspectrophotometric investigations of the dye preparations demonstrated that, besides monomers and dimers, mainly higher AB aggregates are bound to DNA by electrostatic and hydrophobic interactions. These DNA-AB complexes are insoluble in water. Therefore it was possible to stain the DNA-AB films with aqueous EY solutions and also to prepare insoluble DNA-AB-EY films in the second step of the staining experiments. After the reaction with EY, thin sites within the dye preparations were purple. The microspectra of the purple spots show a strong Romanowsky band at 18,100 cm-1. Using a special technique it was possible to estimate the composition of the purple dye complexes. The ratio of the two dyes was approximately EY:AB approximately 1:3. The EY anions are mainly bound by hydrophobic interaction to the AB framework of the electrical neutral DNA-AB complexes. The EY absorption is red shifted by the interaction of EY with the AB framework of DNA-AB-EY. We suppose that this red shift is caused by a dielectric polarization of the bound EY dianions. The DNA chains in the DNA-AB complexes can mechanically be aligned in a preferred direction k. Highly oriented dye complexes prepared on microslides were birefringent and dichroic. The orientation is maintained during subsequent staining with aqueous EY solutions. In this way we also prepared highly orientated purple DNA-AB-EY complexes on microslides. The light absorption of both types of dye complexes was studied by means of a microspectrophotometer equipped with a polarizer and an analyser. The sites of best orientation within the dye preparations were selected under crossed nicols according to the quality of birefringence.(ABSTRACT TRUNCATED AT 400 WORDS)