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OBJECTIVES To measure the concentrations of organochlorine (OC) pesticides in nursing mothers in Western Australia and to evaluate the safety of these concentrations for breast-fed infants; to evaluate the need for future monitoring of OC pesticide concentrations and to determine whether breast milk is an accurate substitute for adipose tissue in future monitoring programs. DESIGN Two cohorts of nursing mothers were recruited during the period October 1990 to March 1991. Levels of OC pesticides were measured in 128 samples of breast milk and 31 samples each of adipose tissue, maternal blood and cord blood. Laboratory analysis was by gas chromatography with electron capture. Health Department studies since 1974 were collated and levels of OC pesticides compared over time. RESULTS DDT (dichlorodiphenyltrichloroethane), HCB (hexachlorobenzene) and dieldrin were found in all samples of breast milk and adipose tissue. A significant fall in the levels of OCs was noted since the 1974 survey, with the current concentrations of the cyclodienes (heptachlor, chlordane and dieldrin) being close to the limit of detection of the assay. The acceptable daily intake for dieldrin was exceeded in 90% of infants and for heptachlor in 2% of infants. The daily intakes of the other OCs were below the respective acceptable levels. HCB was detected at a median level of 0.1 mg/kg in both breast milk and adipose tissue despite being deregistered in 1972. As there is no current acceptable daily intake for HCB, the safety of this level could not be assessed by this method. A low correlation was found between levels of the cyclodiene pesticides in breast milk and adipose tissue, but levels of DDT and HCB were closely correlated. CONCLUSIONS Restrictions on the use of the OC insecticides (DDT, aldrin, dieldrin, heptachlor and chlordane) have resulted in reduced concentrations of these chemicals in breast milk and adipose tissue as compared with previous studies. The dieldrin intake of breast-fed infants consistently exceeded the acceptable daily intake; the heptachlor intake exceeded the acceptable daily intake in some infants, but the low concentrations of heptachlor in breast milk made accurate measurement difficult. The concentrations of DDT and chlordane in breast milk did not pose a hazard to breast-fed infants. The significance of the levels of the fungicide HCB in breast milk cannot be determined. The widespread distribution of HCB, its marked persistence in the environment and its potential for human toxicity indicate a need for continued monitoring, for which breast milk is a suitable medium. The poor correlation between concentrations of dieldrin, heptachlor and chlordane in breast milk and adipose tissue may reflect the technical difficulty of measuring chemical concentrations close to the limit of detection. As aldrin is a registered pesticide and the dieldrin intake of breast-fed infants is consistently above the acceptable daily intake, monitoring of dieldrin should continue. Further population monitoring of DDT, heptachlor and chlordane is unlikely to be of value.