The wild-type strain and four DNA repair-deficient strains (uvrA6, uvrB5, recA56, and polAl) of Escherichia coli K-12 were treated with acetic acid, lactic acid, and p-amimobenzoic acid at pH 3.5 during their stationary phase of growth. All three acids were highly toxic to the polymerase-deficient strain. The greater sensitivity of the strain carrying the poLI gene than its isogenic pol+ derivatives suggested that damage caused by acidity requires polU gene products for repair.