Six lactoperoxidase tolerant Escherichia coli transposon mutants isolated and characterized in an earlier study, and some newly constructed double mutants, were subjected to peroxide, superoxide and hypochlorite stress, and their inactivation was compared to that of the wild type strain MG1655. Knock out mutants of waaQ and waaO, which owed their lactoperoxidase tolerance to an impaired outer membrane permeability due to a reduced porin content, also exhibited higher resistance to hypochlorite, as did a knock-out strain of lrp, encoding a regulatory protein affecting a wide range of cellular functions. Unlike the outer membrane mutants however, the lrp strain was also more resistant to t-butyl hydroperoxide, but more susceptible to the superoxide generating compound plumbagin. Finally, a lactoperoxidase tolerant knock-out strain of ulaA, involved in ascorbic acid uptake, did not show resistance to any of the other oxidants. The possible modes of action of these different oxidants are discussed. (c) 2005 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies.