This paper presents the results of a systematic study on the effects of substituents on the base and sequence specificity of tricyclic heteroaromatic compounds interacting with DNA by intercalation. All the compounds tested are derived from proflavine and acridine orange analogs with different heteroatoms in the middle ring. Their base and sequence specificities were determined by differential dialysis of the ligand against DNA samples of differing G-C content. The main results indicate that (a) the introduction of a phenyl substituent into one of the two available positions of the middle ring increases or decreases the G-C specificity of the ligand depending on the position where the substitution takes place; (b) compounds of the substitution type of neutral red (2-methyl-3-amino-7-dimethyl-amino-phenazine) show unexpectedly high G-C specificities and (c) DNA ligands of pronounced sequence specificity for adjacent G-C pairs can be constructed by combining the structural elements of neutral red with an additional phenyl residue in the same molecule. The further study of compounds related to the phenylated neutral red revealed that the G-C specificity can be improved or destroyed by additional substituents. The comparison of the G-C specificity and the DNA-affinity data of the compounds studied leads to the suggestion that the specificity arises mainly from electronic factors which are strongly controlled through steric constraints on possible ocmplex geometries. As a basis for the discussion a possible structure for the DNA complex of the phenylated neutral red is considered in which the extra phenyl ring at N-5 of the phenazinium system, protrudes into the large groove of the DNA helix while the tricyclic part of the ligand is inserted between the DNA base-pairs.