Extremely small doses of TCDD have been shown to induce hepatic microsomal enzymes in animals. Whether levels of environmental exposure to TCDD were sufficient to produce enzyme induction in man, has been investigated in Seveso, where in July 1976 explosion in a factory spread toxic substances, one of which was TCDD, to the surrounding area. The hepatic microsomal enzyme activity was assessed by estimating urinary d-glucaric acid (UGA) excretion in children 6-8 years old. In 31 children, urine samples were collected between August and December 1976; in 67 other children in February 1979. As a control group 60 children living in Busto Arsizio (a small industrial town near Milan) and 26 living Cannero (a non-industrialized village on Lake Maggiore) were chosen. In the first period of collection, children with chloracne (which is considered to be a characteristic manifestation of intoxication with chlorinated products), showed significantly increased levels of UGA compared with children without chloracne. In 1979, children living in the Seveso area showed a statistically significant enhancement of d-glucaric acid excretion compared to the control groups. In conclusion, this study demonstrates that many children living in the Seveso area have an increased activity of hepatic microsomal enzymes, since, although the urinary excretion of d-glucaric acid is only an indirect measure of enzyme activity, studies in man have indicated that it is both sensitive and quantitative. As far as the cause of this increase is concerned, since it is possible to exclude the influence of alcohol, contraceptives, phenobarbitone or other drugs, it is reasonable to conclude that TCDD, a potent inducer agent, could be responsible for this phenomenon.