Lead is known to have toxic effects on heme synthesis, being an especially potent, and, relative to other metals, specific inhibitor of the enzyme δ-aminolevulinate dehydratase (ALAD). The mechanism of inhibition of ALAD by lead is uncertain, but binding of lead to ALAD sulfhydryl groups has been assumed. Mechanisms involving lead interaction with the enzyme substrate (ALA) rather than the enzyme are also reasonable, but have not as yet been considered; therefore the interaction of lead acetate with ALA was investigated by carbon-13 NMR spectroscopy, a technique shown to be very powerful in examining ligand-metal interactions. Addition of lead acetate solution to 0.25 m ALA at pH 6.5 resulted in a pronounced shift of certain of the carbon resonances of the latter, a maximal effect occurring at an ALA: Pb concentration ratio of 2. No similar effects were observed with acetates of calcium, zinc, cadmium, silver, or mercury. Studies on the variation in shift of resonances from invididual carbon atoms with lead acetate concentration suggested that lead interacted most strongly in the region of the carboxyl and α-carbon atoms of ALA. The variation of shift with pH showed that at an ALA: Pb ratio of 1 the p K a of the ALA carboxyl group was decreased by 1.2 units. These studies demonstrate that lead is specific among the metals tested in forming a complex with δ-aminolevulinic acid. Such complex formation may be important in the mechanism of lead toxicity on the heme biosynthetic pathway.