To determine the potential for using urine instead of blood as an indicator of lead exposure, especially in infants, lead concentrations and high-precision lead isotopic measurements have been compared in venous blood and 'spot' urine (n>260 from 182 different subjects) collected within the same 24-h period. Physiological conditions for the children and most of the adults were considered to be in a steady-state between body stores and lead in the environment. In the case of some adults, conditions were initially not steady-state because exposure conditions changed (for example, subjects moved to a country with lead of different isotopic composition.) There was a high correlation (r2 = 0.98) between the blood and urine measurements of the isotope ratios but about 10% of measurements were outliers-the blood and urine measurements were further apart than was consistent with the measurement error that was generally obtained. The discrepancy was usually found to be associated with the urine measurement and was attributed to contamination during sampling. Weekly urine and monthly blood monitoring of an adult male over a 24-month period showed excellent correlations, although the standard deviations were about an order of magnitude higher than the precision measured for replicate analyses of a single blood or urine sample. 'Spot' urine analyses for two male subjects gave excellent agreement with 24- h urine samples. Standard deviations of the spot analyses were of similar order to those in the 24-month monitored subject. In cases where female adults from Eastern Europe migrated to Australia, there was generally a more rapid exchange of skeletal lead with Australian environmental lead in urine compared with blood. These data do not support a differential partitioning of endogenous lead into the plasma. At this stage, isotopic measurements of urine can be used as a proxy for isotopic measurements in blood. However, lead concentrations in blood and in urine are only weakly related. Concentrations of lead in urine cannot serve to predict concentrations of lead in blood, particularly at the lower range of exposures, for example, at blood lead concentrations less than 10 μg/dl.