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A comparative study of the ability of amino-substituted benzo[b]quinolizinium derivatives to act as DNA- or protein-sensitive fluorescent probes is presented. Spectrophotometric titrations, DNA denaturation studies and viscometric titrations showed that all tested aminobenzo[b]quinolizinium derivatives intercalate into DNA with binding constants K_b = 10⁴-10⁵ M⁻¹. The intense fluorescence of the 9-aminobenzo[b]quinolizinium (Φ_fl = 0.41) as well as the intrinsically very weak emission of the 7-aminobenzo[b]quinolizinium (Φ_fl < 0.005) are quenched by the addition of DNA, most likely caused by a photoinduced electron transfer (PET) between the excited intercalated ligand and the DNA bases. The 6-aminobenzo[b]quinolizinium (1b) and the 6-amino-9-bromobenzo[b]quinolizinium (1c) exhibit very low fluorescence intensity in water (Φ_fl < 0.005). However, in water-glycerol mixtures the emission intensity …