Toxicology studies were conducted with the mono-, di-, and pentachlorophenols (CP). Chlorophenols (except PC) demonstrate a relatively low order of toxicity. The order of toxicity in mice and rats (most to least) is: PCP > tetra CPs > mono CPs > tri CPs > di CPs. Short-term (14 days) repeated exposure to 2-CP at (gavage) doses of 35, 69, or 175 mg/kg/day (approximately 1/10, 1/5, 1/2 the acute oral LD50) resulted in 100% lethality at the highest dose and no biologically significant compound related effects at the lower doses. Short-term (14 days) repeated exposure to 2,4-DCP at (gavage) doses of 64, 128, or 638 mg/kg/day (approximately 1/20, 1/10, 1/2 the acute oral LD50) failed to induce significant compound related toxicity. The liver was identified as a possible target organ. Subchronic exposure to 2,4-DCP at drinking water concentrations of 0.2, 0.6, and 2.0 mg/ml (limit of solubility and acceptability was 2.0 mg/ml) for 90 days (approximate doses of 50, 150, 500 mg/kg/day) failed to induce significant compound related toxicity. The data presented and a review of the available literature support the relatively low order of toxicity of most chlorinated phenols ingested orally (except PCP).