A principal target for the environmental toxin lead (Pb) is porphobilinogen synthase (PBGS), a Zn-metalloenzyme necessary for heme biosynthesis. Measurement of blood Pb inhibited PBGS is the most sensitive indicator of subclinical Pb intoxication, but problems with the assay have diminished its use. This report identifies Pb as a slow acting inhibitor of PBGS. The activity of PBGS could change up to sixfold during an hourlong clinical assay of Pb contaminated blood, and activity is profoundly effected by the presence of serum proteins, such as albumin. When PBGS catalyzed PBG production is allowed to reach a steady state rate, kinetic data on purified PBGS support the hypothesis that Pb inhibition of PBGS results from direct substitution for Zn.