Commercially obtained pure human serum albumin (HSA) was shown to contain molecular aggregates and was significantly contaminated with Cu(II). A solution of commercial HSA was first passed through a Sephadex G-200 column to obtain pure monomeric HSA. The monomer of HSA was subsequently passed through Chelex-100 resin to free it from Cu(II). All Cu(II)-binding studies were conducted with monomeric and copper-free HSA. The first Cu(II)-binding site on HSA appears to be stronger than the second and the subsequent binding sites. Significant amounts of L-histidine and L-threonine were bound to HSA when Cu(II) was added in the form of Cu(II) – amino acid complexes. In the absence of Cu(II), free L-histidine or L-threonine do not bind to HSA at pH 7.4. It is concluded that, in the presence of either L-histidine or L-threonine, ternary complex formation is involved both at the first and the subsequent binding sites for Cu(II) on HSA. In view of this finding, it appears that the equilibrium between HSA–Cu(II) and Cu(II) – amino acid complex is mediated through a ternary complex HSA – Cu(II) – amino acid.