Various synthetic derivatives of phenothiazines, benzo[a]phenothiazines and benz[c]acridines were compared for their abilities to induce antiplasmid activity against E. coli F'lac plasmid. Several phenothiazine derivatives were much more potent in antiplasmid activity than benzo[a]phenothiazine- or benz[c]acridine derivatives. Their antiplasmid activity seemed to be enhanced by Cl- or CF3- substitution at 2 C atom, and modified by the side chain length and charge at the L-region of the molecules, as well as by hydrophilicity.