A series of nitro compounds studied inhibited Eecherichia coli, ~taphylococcua aureue and P 8 e U d O V l O W aeruginosa, bronopol being the mout active derivative. Bronopol causes some membrane damage. A consideration of its action on enzyme activity, reaction with cell constituents and chemical reaction with cysteine, glutathiono and proteins suggests that the oxidation of thiol groups t o disulphides may be its primary action. ZSOLNAI (1961) examined a number of alkyl and aralkyl nitro compounds and demonstrat,eci the antifungal activity of this group, particularly where the structural feature, CBrNO,, was present. Eckstcin et al. (1963) described the antifungal activity of some 2-nitropropan-l,3-01 derivatives and showed that 2-bromo-2-nitropropan-1,3 tliol was one of the more active compounds. Croshaw, Groves & Lessel (1964) studied the activity of 2-bromo-2-nitropropan-l,3-diol (bronopol) and found it to be active against both Gram positive and Gram negative bacteria, being particularly active against Pseudomonas aeruginosa, and exhibiting activity against yeasts and fungi. The bacteriostatic action was decreased in the presence of cysteine hydrochloride, serum or horse blood. The wide spectrum of activity makes it suitable for use as a preservative in pliarmaceutical products (Sykes, 1969), and because of its lack of irritancy combined with stability in aqueous solution, Bryce & Smart (1965) recommended its use in shampoo formulations. Brown (1966) showed that bronopol was rapidly active against Ps. aeruginosa and unaffected by the presence of nonionic detergents. Materials and Methods OrganismsEscherichia coli NCTC 9001 , Staphylococcus aureus NCIB 8626 and Pseudomom aeruginosa NCIB 8295 were used throughout, and Bacillus megaterium NCIB 9521 for the preparation of protoplasts. MediaThe organisms were maintained in a nutrient broth containing (g/l): Oxoid beef extract, 10; Oxoid neutralized peptone, 10; sodium chloride, 6. Solid media were * Present address, Rexford Ltd, Manningtree, Essex. 1611 62 R. J. Stretton and T. W. Manson prepared by adding 1.5% w/v of Oxoid agar No. 3. A drug neutralizing medium was prepared by adding 0-1 1 yo of sodium thioglycollate. The pH value of all media was adjusted to 7.2. Chemicals Bronopol (2-bromo-2-nitropropan-l,3-diol) is a white, odourless, crystalline solid, map. 121 O, and is readily soluble in water. The sample used was a gift from The Boots Co. Ltd, Nottingham, England. The other diol nitro compounds examined, 2-nitro propan-1,3-diol and 2-chloro-2-nitropropan-l,3-diol were prepared by the method of Eckstein et al. (1963). 2-Dinitropropane, 2,3-nitro-2-methylbutane, 2-bromo-2nitropropane, 2-chloro-2-nitropropane and 2-bromo-2-nitro-3,3-dimethylbutane were prepared by Dr W. R. Bowman of this Department. Sterile solutions of the nitro compounds were prepared by filtration through a membrane filter of 0.2 pm porosity (Sartorius Membrane filter, V.