The acute toxicity of the antioxidant 2,6-di- tert-butyl-4-hydroxymethylphenol (Ionox 100) has been determined for rats and mice. Chronic toxicity, including carcinogenicity, and a three-generation reproduction study has also been carried out using rats. It was found impossible to determine the acute oral LD50 for rats and mice by a single-dose technique (LD50 > 7 g/kg). The compound significantly increased the growth rate of rats over their life span when fed at dose levels of 0.2% and 0.35% in the diet. No significant differences were found between the control and treated rats for the following parameters: food intake, survival, terminal organ weights and pathology, hematology and histopathology. There were no effects on the growth rate, survival, fertility, and litter sizes when measured over 3 successive generations of animals. A no-effect level of 0.35% Ionox 100 was established in the diet of rats, a level equivalent to 175 mg/kg/day. An estimate of the acceptable daily intake for man therefore is up to 1.75 mg/kg of body weight.