Soil profiles from an urban-rural transect near Adelaide, South Australia, have been analyzed by using lead isotopes as a discriminant for the source of soil lead. Analyses of the top 10 cm of soil profiles taken from sites up to 50 km from Adelaide show that this surface horizon mainly contains Precambrian lead, such as that found in the Broken Hill or Mount Isa deposits. There is also a small amount of younger lead present, similar to that derived from Missouri and young Canadian deposits. Both types of lead are used in the manufacture of tetraethyllead for Adelaide gasoline. Natural soil lead, derived from parent rock, occurs throughout the profiles; however, it is only a relatively minor component in the surface horizons but is the major component at depths greater than 30-40 cm. Assessment of the data suggests that orchard sprays, power stations, and smelters are not the main source of lead in the surface horizons - rather, contamination derived from tetraethyllead from gasoline is indicated.