Palladium(II) complexes of phenothiazines were synthesized and examined for their antiplasmid activity on E. coli. It was found that the complex of unsubstituted phenothiazine was ineffective; however, the complexes of chlorpromazine and promazine were able to eliminate the F'lac plasmid of E. coli and inhibited intercellular plasmid transfer. The crystal structure of [Pd(pzH)Cl3] (pzH is protonated promazine) has been determined by X-ray diffraction methods and refined to a residual of 0.028 for 2821 independent observed reflections. The crystals are monoclinic, space group P21/n; a 8.166(2); b 26.855(5); c 9.141(2) Å; β 110.65(2)°. Promazine is bonded via an S atom to a PdCl3- unit with the charge being balanced by a protonated amino group.