Detailed studies are reported on the Ni(II)-binding site of human serum albumin (HSA) and the results are compared with those obtained from the N-terminal native-sequence peptide, l-aspartyl-l-alanyl-l-histidine N-methylamide (Asp-Ala-His-NHMe). Equilibrium dialysis of HSA and Ni(II) in 0.1m-N-ethylmorpholine/HCl buffer, pH 7.53, demonstrates a specific Ni(II)-binding site on the protein. l-Histidine, the low-molecular-weight Ni(II)-binding constituent of human serum, is shown to have a greater affinity for Ni(II) than does HSA. A small but significant amount of ternary complex HSA-Ni(II)-l-histidine is also present in the equilibrium mixture containing the three components. The log (association constant) values for the binary and ternary Ni(II) complexes are 9.57 and 16.23 respectively. The complex equilibria between Asp-Ala-His-NHMe and Ni(II) have been investigated by analytical potentiometry in aqueous solution (0.15m-NaCl, 25 degrees C). Several species, including MA, MA(2), MH(-2)A, and MH(-1)A(2) [where M and A represent Ni(II) ion and anionic peptide respectively], were detected in the system, MH(-2)A being the major complex species. Equilibrium studies involving Asp-Ala-His-NHMe, Ni(II) and l-histidine reveal the presence of a ternary complex MH(-1)AB (where B represents anionic l-histidine) at physiological pH. Detailed studies of visible-absorption spectra of HSA in the presence of Cu(II) and Ni(II) reveal that the two metal ions bind HSA at the same site. The visible-absorption spectrum of Ni(II)-HSA complex shows a highly absorbing peak at 420nm (epsilon(max.) = 137; with shoulder at 450-480nm) characteristic of a square planar or square pyramidal co-ordination arrangement about the metal ion. Similar visible-absorption characteristics were observed for the major species MH(-2)A in the Asp-Ala-His-NHMe-Ni(II) system (lambda(max.) = 420nm; epsilon(max.) = 135; with shoulder at 450-480nm). The combination of experimental results from the protein studies and the peptide analyses provides strong evidence for the structure of the Ni(II)-binding site of HSA as one that involves the alpha-amino nitrogen atom, two deprotonated peptide nitrogen atoms, the imidazole nitrogen atom and the side-chain carboxy group of the aspartic acid residue. On the basis of the results obtained from the individual ternary systems involving protein and peptide, a mechanism for the transportation of Ni(II) in the serum is proposed.