We have examined in vitro the antioxidant properties of 2,6-diisopropylphenol. In studies using electron spin resonance spectroscopy we have demonstrated that 2,6-diisopropylphenol acts as an antioxidant by reacting with free radicals to form a phenoxyl radical--a property common to all phenol-based free radical scavengers. In additional experiments, the antioxidant properties of the clinical formulation of 2,6-diisopropylphenol (propofol) have been measured using an assay of antioxidant potential. In these experiments, propofol, but not Intralipid, was found to exhibit significant antioxidant activity, such that in the range of propofol concentrations examined (10(-6)-10(-5) mol litre-1), each molecule of 2,6-diisopropylphenol was able to scavenge two radical species. We conclude that the free radical scavenging properties of 2,6-diisopropylphenol resemble those of the endogenous antioxidant alpha-tocopherol (vitamin E).