We have obtained stable lead isotope and lead concentmtion data from a longitudinal study of mobilization oflead from the maternal skeleton during pregnancy and lactation and in which the newly born infants were monitored for 6 months postpartum to evaluate the effects of the local environment on lead body burden of the infant. Samples of maternal and infant blood, urine, and diet and especially breast milk were measured for 21 mothers and 24 infants. Blood lead concentrations were less than 5 pg/dl in all except one subject. The mean lead concentration in breast milk ± standard deviation was 0.73 ± 0.70 p/kg. In seven subjects for whom serial breast milk sampling was possible, the lead concentration varied by factors of from 2 to 4, and for three subjects there was an increase at or after 90 days postpartum. For the first 60-90 days postpar-tum, the contribution from breast milk to blood lead in the infnts varied from 36 to 80%. Multiple linear regression analyses indicated statistically significant relationships for some of the variables ofisotope ratios and lead concentrations between breast milk blood, urine, and diet for infants and mothers. For example, the analyses revealed that both a mother's breast milk 207Pb/206Pb and 206Pb/204Pb ratios and lead concentration provide information to predict her infant's blood 207Pb/206Pb and 206Pb/204Pb ratios. The major sources of lead in breast milk are from the maternal bone and diet. An evaluation of breast milk lead concentrations published over the last 15 years indicates that studies in which the ratio of lead concentrations in breast milk to lead concentrations in whole maternal blood (x100) were greater than 15 should be viewed with caution because of potential contamination during sampling and/or laboratory analyses. Selected studies also appear to show a linear relationship between breast milk and maternal whole blood, with the percentage of lead in breast milk compared with whole blood of <3% in subjects with blood lead levels anging from 2 to 34 p/di. The levels of lead in breast milk are thus similar to those in plasma. Breast-fed infants are only at risk if the mother is exposed to high concentrations of contaminants either from endogenous sources such as the skeleton or exogenous sources.

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