Ditercalinium (NSC 366241), a dimer of 10-methoxy-7H-pyrido[4,3-c]carbazole quaternarized on the pyridine nitrogen by a rigid bis(1,1'-ethyl)-4,4'-bipiperidine linking chain, is endowed with antitumour properties and bis-intercalates with high affinity into DNA. New dimers have been designed in the same series to evaluate the importance of the rigidity of the linking chain for pharmacological activity. The dimers, characterized by one and two additional methylene groups between the two piperidine rings of the linking chain, remain as active as ditercalinium. However, a third additional CH2 group between the two piperidine rings leads to an inactive dimer. Relationships between the different pharmacological activities of the drugs and their intercalation complexes with DNA were investigated using viscosimetry, absorption spectroscopy and NMR analyses.