The effect of toluene on Escherichia coli has been examined. In the presence of Mg2+, toluene removes very little protein, phospholipid, or lipopolysacharide from E. coli. In the absence of Mg2+, or in the presence of EDTA, toluene removes considerably more cell material, including several specific cytoplasmic proteins such as malate dehydrogenase (EC 1.1.1.37). In contrast, glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and glutamate dehydrogenase (EC 1.4.1.4) are not released at all under the same conditions. Cells treated with toluene in the presence of Mg2+ remain relatively impermeable to pyridne nucleotides, while cells treated with toluene in the presence of EDTA become permeable to these compounds. Freeze-fracture electron microscopy shows that toluene causes considerable damage to the cytoplasmic membrane, while the outer membrane remains relatively intact. These results indicate that the permeability characteristics of toluene-treated cells depend at least partly on the state of the outer membrane after the toluene treatment.