The Hg-199 NMR spectra of spinach plastocyanin and Pseudomonas aeruginosa azurin have been measured after substitution of Hg(II) into the blue copper-binding sites. The chemical shifts for plastocyanin and azurin reflect a primary coordination environment consisting of two histidines and a cysteine thiolate and, in the case of plastocyanin, an axial methionine. The sensitivity of Hg-199 chemical shifts to the primary and secondary coordination environments of mercury complexes makes Hg-199 NMR a powerful spectroscopic probe of metal-binding sites in complex ions and metalloproteins.