In exposure and risk evaluation, monitoring lead biologically has several advantages over technical exposure assessment. Traditionally, the concentration in blood (B-Pb) has been widely used. However, the erythrocytes tend to become saturated, and this phenomenon causes a nonlinear relationship between B-Pb and uptake and between metabolic and toxic effects and B-Pb. Recently, several techniques for determining lead in finger bone, tibia, or calcaneus in vivo by X-ray fluorescence have become available. Bone lead reflects long-term exposure and should prove valuable in epidemiologic studies. Mobilization tests have been widely used to monitor lead biologically. They mainly seem to reflect the lead in soft tissues and may not be an index of total body burden, most of which is in the skeleton. It thus seems that, at least in adults, mobilization tests do not provide more information than traditional lead determinations in blood and urine. A metabolic model for lead in humans is presented.