Public health and medical recommendations on prevention of lead toxicity rely on use of blood lead concentrations to assess lead exposure and predict onset of adverse health effects. Blood lead levels have generally been thought to reflect recent environmental lead exposures. However, tissue lead stores are accumulated over a long time period (i.e., years). These tissue stores, primarily from bone, can be remobilized as part of both normal physiologic and pathologic processes. Although chemical analyses do not differentiate lead isotopes, mass spectrometric determinations can differentiate the quantities of stable lead isotopes present in particular samples (e.g., lead 207, lead 206, lead 204, and lead 208). Selected geographic locations may have distinct isotopic profiles. For example, on mainland Australia the 206Pb/204Pb ratios reported in both environmental lead sources and blood samples are typically less than 17.0. By contrast, stable lead isotope profiles in blood samples of adult women immigrating from Eastern Europe and the former Soviet Union usually have 206Pb/204Pb ratios greater than 17.5 and as high as 18.5 on entry into Australia. Longitudinal monitoring of blood samples to determine stable lead isotope profiles by mass spectrometry and chemical analyses of blood samples for total lead content were conducted over a 300-day period. These data show that between 45% and 70% of lead in blood comes from long-term tissue lead stores. Recognition that the predominant source of lead in blood was tissue stores rather than the contemporaneous environment should greatly modify recommendations on use of blood lead to monitor occupational or environmental interventions. In addition, internal biokinetics of lead, documented through presence of tissue lead in blood, underlie the long-term health risks of lead exposure. Transfer of lead to the fetus from maternal tissue stores represents a special area of concern.