The question of the extent of lead mobilization from the maternal skeleton during pregnancy and lactation is one of the most outstanding problems of lead toxicity. We have undertaken a longitudinal cohort study in an urban environment of European female immigrants of child-bearing age (18 to 35 years) to Australia whose skeletal lead isotopic composition has been determined to be different from that in their current environment. The cohort was to consist of 100 immigrants anticipated to provide 20 pregnant subjects who would be compared with two groups of control subjects: a matched immigrant nonpregnant control group and second-generation Australian pregnant control subjects. Pregnant subjects also serve as their own controls for a comparison of changes during gestation with those before conception. High-precision lead isotopic compositions and lead concentrations are measured in maternal blood and urine prenatally, monthly during gestation, and postnatally for 6 months; they are also measured in infant blood and urine for 6 months; environmental measures are sampled quarterly for 6-day duplicate diet, house dust and water, and urban air and gasoline. Because of continuing public health concerns about lead exposure, interim findings from this cohort are being reported. To date there have been 13 conceptions in immigrant subjects, with 7 births, in addition to 3 conceptions in the Australian control group, with 2 births. PbBs have been generally low, with a geometric mean of 3.0 microg/dl, and have ranged from 1.9 to 20 microg/dl. Increases in PbB of approximately 20% during pregnancy have been detectable even in subjects with low blood lead levels. The skeletal contribution to blood lead level, based on isotopic measurements, has exhibited a mean increase (and standard deviation) of 31% +/- 19% with a range from 9% to 65%. Earlier studies that used lead concentrations only have suggested that blood lead levels increased only during the second half of pregnancy. This increase in blood lead levels has also been observed in the present study. However, in two subjects the increases in total blood lead were also detected in the first 2 months of pregnancy. Changes in isotopic composition and blood lead during gestation for Australian pregnant controls were negligible. The ratio of cord/maternal blood lead levels varied from 0.54 to 1.05, and the ratio for the isotopic composition was 0.993 to 1.002. Results of this study confirm that lead is mobilized from skeletal stores at an accelerated rate during pregnancy and is transferred to the fetus. These results also show that mobilization from long-term stores (i.e., bone) contributes significantly to blood lead levels during pregnancy. Furthermore, exposure of the fetus to lead during pregnancy has implications for interpretations of neurobehavioral disorders attributed to only postnatal exposure. Even after 800 days of residence in Australia, the contribution of European skeletal lead to blood lead in nonpregnant subjects can be on the order of 50%, but the current PbB may give no indication of the former high skeletal lead burden.