Chlorinated phenols, which are used primarily as wood preservatives and fungicides, are present in most air, water, and soil samples in industrialized areas as well as in urine and body fat of most people. The ability of phenol and the 19 isomers of chlorophenol to induce DNA damage using the Microscreen prophage-induction assay in Escherichia coli has been examined. Seven of the isomers (2,3,4-tri, 2,4,5-tri, 3,4,5,-tri, 2,3,4,5-tetra, 2,3,6-tri, 2,4,6-tri, and pentachlorophenol) induced prophage lambda in the presence of S9, with the first three being 10 times more potent than the last three. The more potent isomers have either one or no chlorine atom ortho to the OH group; whereas the less potent isomers have two chlorine atoms ortho to the OH group. Although none of the 20 compounds is mutagenic in Salmonella, the prophage-induction results agree with findings by others that most of these seven isomers are clastogenic, are associated with cancer and chromosomal aberrations in humans, and are carcinogenic in rodents. A likely basis for the genotoxicity of the seven isomers involves the metabolism of the parent isomer to chlorohydroquinone, which can form a chlorobenzosemiquinone in the presence of oxygen. (Copyright (c) 1990 Wiley-Liss, Inc.)