The document provides the health effects basis to be considered in establishing the Maximum Contaminant Level Goals (MCLGs) for four trihalomethanes found in chlorinated drinking water. To achieve this objective, data on pharmacokinetics, human exposure, acute and chronic toxicity to animals and humans, epidemiology and mechanisms of toxicity were evaluated. Specific emphasis is placed on literature data providing dose-response information. While the literature search and evaluation performed in support of the document was comprehensive, only the reports considered most pertinent in the derivation of the MCLGs are cited in the document. The comprehensive literature search in support of the document includes information published up to 1989, however, more recent information may have been added during the review process.