Here, we report that lead-thiolate charge-transfer bands (250-400 nm) can be used to monitor lead binding to cysteine-rich sites in proteins and report the application of this technique to determine the thermodynamics of lead binding to a series of structural zinc-binding domains. These studies reveal that Pb2+ binds tightly to structural zinc-binding domains with dissociation constants that range from Kd(Pb) = 10(-9) to 10(-14) M, depending on the number of cysteine residues in the metal-binding site. Competition experiments with Zn2+ lead to two striking conclusions: first, the two metals rapidly equilibrate, and second, the ratio of Pb2+ to Zn2+ bound to a particular site is determined by the relative affinities of the two metals for that site, rather than being under kinetic control. We conclude that Pb2+ should be able to compete effectively with Zn2+ for Cys(4) sites under physiological conditions. Despite the fact that Pb2+ binds tightly to cysteine-rich structural zinc sites, circular dichroism and H-1 NMR studies reveal that Pb2+ does not stabilize the correct fold of the peptides.