Summary.A series of nitro compounds studied inhibitedEscherichia coli, Staphylococcus aureusandPseudomonas aeruginosa, bronopol being the most active derivative. Bronopol causes some membrane damage. A consideration of its action on enzyme activity, reaction with cell constituents and chemical reaction with cysteine, glutathione and proteins suggests that the oxidation of thiol groups to disulphides may be its primary action.