The effects of in vivo exposure to Pb2+ on avian aminolevulinic acid dehydratase (ALA-d) activity were studied, and the activity ratio (activated/non-activated enzyme activity) was assessed as a potential indicator of Pb exposure in birds. A significant negative correlation was observed between blood-Pb concentration and log ALA-d activity (r = -0.827) in Pb-dosed birds. An even better correlation was obtained when data were expressed as an activity ratio (r = 0.961), demonstrating that sample variability is decreased by expressing the results as an activity ratio rather than as enzyme activity. Red blood cell (RBC) ALA-d activity ratio was a sensitive, dose-responsive measure of Pb exposure regardless of the mode of administration of Pb. Dietary Pb concentrations as low as 5 ppm (dry wt) could be estimated through the use of the activity ratio method, and a highly significant positive correlation was observed between dietary Pb concentration over the 5-100 ppm range and the ALA-d activity ratio (r = 0.87). The RBC-ALA-d activity ratio may thus be a useful method for estimating average dietary concentrations of Pb over an environmentally relevant range in situations where the diet is the major source of exposure to Pb, and where it is also impossible or impractical to accurately measure dietary Pb content directly.