Plasmid elimination in Escherichia coli by a quaternary amine of chlorpromazine was demonstrated on different incompatibility groups of plasmid. The biological effect of the drug depends partly on the host bacteria and partly on the plasmid itself. Various receptor substrates such as adenosine, dopamine, histamine and norepinephrine do not alter the plasmid elimination by promethazine and imipramine. None of the known drug-receptors studied are involved in drug binding of the bacteria. The direct membrane action of imipramine and promethazine was demonstrated in electron microscopic studies and alterations in the bacterial membrane such as discontinuities, phase separation or rarely extensive lytic alterations were observed. Magnesium ions prevent the ultrastructural membrane alterations caused by imipramine and promethazine. There is some evidence that the drugs bind to two different receptor sites simultaneously on the plasmid replication site. The first and strongest binding has to be ionic through the side chain amino group, displacing the bivalent cations. In turn, the two aromatic rings of the fixed (ionically bound) drug molecules bind weakly through pi-electrons, hydrophobically or by a charge transfer complex. This weaker binding together with the ionic one are essential for biologic action and lead to the inhibition of plasmid replication. A schematic model of the effect of tricyclic psychotropic drugs on the bacterial membrane is proposed.