H-1 and Cd-113 NMR studies are used to investigate the Cd2+ binding sites on serum albumin (67 kDa) in competition with other metal ions. A wide range of mammalian serum albumins possess two similar strong Cd2+ binding sites (site A 113-124 ppm; site B 24-28 ppm). The two strong sites are shown not to involve the free thiol at Cys34. Ca2+ influences the binding of Cd2+ to isolated human albumin, and similar effects due to endogenous Ca2+ are observed for intact human blood serum. H-1 NMR studies show that the same two His residues of human serum albumin are perturbed by Zn2+ and Cd2+ binding alike. Zn2+ displaces Cd2+ from site A which leads to Cd2+ occupation of a third site (C, 45 ppm). The N-terminus of HSA is not the locus of the two strong Cd2+ binding sites, in contrast to Cu2+ and Ni2+. After saturation of the N-terminal binding site, Cu2+ or Ni2+ also displaces Cd2+ from site A to site C. The effect of pH on Cd2+ binding is described. A common Cd2+/Zn2+ binding site (site A) involving interdomain His residues is discussed.