Newly born infants (n = 15) were monitored for 6 months after birth or for longer periods to evaluate the changes in isotopic composition and lead concentration in infants as compared with that in women from the same population groups and to determine the clearance rates of lead from blood in the infants. These data represent the first published results for serial blood sampling in a relatively large cohort of newly born infants. Blood lead concentrations decrease from the cord to samples taken at 60 to 90 days and then increase by amounts varying from negligible to 166%. In spite of concern about individual susceptibility to lead pharmacokinetics, changes in isotopic ratio followed a smooth decrease over time for 9 of the 11 infants born to migrant parents, and the patterns of variation were quite reproducible. Data for 2 of 4 infants born to multigenerational Australian parents exhibited little change in isotopic ratio over time, and in the other two cases, the changes were attributed to diet. The rate of exchange (t(1/2)) for the migrant infants of lead in blood derived from the mother during pregnancy and the lead from the current environment was calculated by using a linear function and ranged from 65 to 131 (91 ± 19, mean ± SD) days. The half-lives for the exchange of skeletal and environmental lead for 7 of the 8 women before significant mobilization of lead from the maternal skeleton ranged from 50 to 66 (59 ± 6) days. One explanation for the longer half-lives for infants as compared With the mothers may be the proportionally higher contribution of current environmental (Australian) lead in the infants at parturition. Exchanges of lead in infants are more complex than for the adults, reflecting inputs from sources such as maternal skeletal lead during breast feeding.