Levels of I were determined in selected foods and dietary supplements, and in samples of the British 'Total Diet'. The average concentration of I in British milk collected in thirteen areas on four occasions during 1990 and 1991 was 150 micrograms/kg (range 40-310 micrograms/kg), compared with 230 micrograms/kg in 1977-79. No difference was found between skimmed and whole milk. Winter milk contained 210 micrograms/kg while summer milk contained 90 micrograms/kg. Regional differences were less pronounced than seasonal differences. Levels in fish and fish products were between 110 and 3280 micrograms/kg. Edible seaweed contained I levels of between 4300 and 2,660,000 micrograms/kg. Kelp-based dietary supplements contained I at levels that would result in a median intake of 1000 micrograms if the manufacturers' recommended maximum daily dose of the supplement was taken, while other I-containing supplements contained a median level of 104 micrograms in the manufacturers' maximum recommended daily dose. Intake of I, as estimated from the Total Diet Study, was 173 micrograms/d in 1985 (277 micrograms if samples with very high I contents were included) and 166 micrograms/d in 1991. These levels are above the UK reference nutrient intake of 140 micrograms/d for adults but well below the Joint Expert Committee on Food Additives provisional maximum tolerable intake of 1000 micrograms/d.