Triethyltin (TET) stimulated the basal respiration of Escherichia coli K-12 membrane vesicles in chloride (Cl-) medium but it had little effect on respiration in sulphate (SO4(2-)) medium. Since this uncoupling activity was Cl- dependent it was attributed to the Cl-/hydroxide (OH-) exchange reaction known to be mediated by TET [1,2]. TET inhibited the oxidation of succinate by intact E. coli in both Cl- and SO4(2-) medium, but at the same concentration of TET, inhibition was always more extensive in Cl- than SO4(2-) medium. In Cl- medium uncoupling in membrane vesicles and inhibition of succinate oxidation in intact bacteria occurred over the same concentration range and it appeared that the same mechanism, i.e. Cl-/OH- exchange, was responsible for both effects. Inhibition of succinate oxidation in SO4(2-) medium was not substantial until the concentration of TET was greater than 10(-5) M. Although the nature of this inhibition could not be determined by experiments with membrane vesicles indirect evidence from growth experiments indicated that it was due to impairment of oxidative phosphorylation. The relationship between these biochemical findings and the bacteriocidal action of TET was examined by using various concentrations of anion and substrate in the growth medium. Growth was inhibited in media containing either Cl- or SO4(2-) as the main anion but at a particular concentration of TET, inhibition was greater in Cl- medium. Growth was also inhibited to a greater extent in succinate than glucose medium. Furthermore in either Cl- or SO4(2-) glucose medium, lactic acid production increased as the concentration of TET was increased. These findings imply that the bacteriocidal action of TET is related to its effect(s) on oxidative phosphorylation.