Some tricyclic psychotropic drugs are known to have plasmid curing activity. The interaction with DNA of three plasmid curing (chlorpromazine, amitriptyline, imipramine) and four ineffective (methylene blue, 7,8-dioxo-chlorpromazine, thiazinamium, chlorpromazine sulphoxide) compounds was studied by fluorescence polarization and circular dichroism methods. Among the seven compounds three, namely chlorpromazine, 7,8-O2-chlorpromazine, and methylene blue showed an intercalation effect. Other phenothiazines such as chlorpromazine sulphoxide and thiazinamium were not able to intercalate into Escherichia coli DNA, neither did the plasmid curing drugs amitryptyline and imipramine. It is concluded that the plasmid curing ability is not necessarily related to the intercalation ability.