Acinetobacter johnsonii has potential use in the remediation of heavy metal-contaminated wastewaters. For metal accumulation, cells must remain intact and metabolically active. The effect of possible accumulation targets, Cd2+, UO22+, Cu2+, and Ni2+ on cytoplasmic membrane integrity and polarity was investigated by flow cytometry using a mixture of two fluorescent dyes, propidium iodide and bis-oxonol. The former binds to DNA but cannot cross an intact cytoplasmic membrane, whilst the latter is anionic, lipophilic and stains depolarized cytoplasmic membranes. All four metals permeabilized the cytoplasmic membrane of some cells during the period of exposure. The effects of the metals differed in that Cu2+ and Cd2+ also generated an intermediate population of cells, having intact but depolarized cytoplasmic membranes. Electron microscopy showed corresponding cellular abnormalities following metal exposure.