Albumin, the major copper-binding protein in blood serum, was shown to form different albumin-copper complexes in in vivo and in vitro. Cupric ions added in vitro to control rat serum bound preferentially to mercaptalbumin and the mercaptalbumin-copper complex remained unchanged with time. Cupric ions injected intravenously into the rat first formed the mercaptalbumin-copper complex; this binary complex changed gradually with time to form an albumin-copper-cysteine complex. The participation of cysteine in the formation of this complex was demonstrated in vitro and further suggested that its conversion was an oxidative reaction. Glutathione also participated in forming the complex, but it was not as effective as cysteine. Albumin-copper complexes were separated on a gel filtration column and detected simultaneously by high-performance liquid chromatography-inductively coupled argon plasma-atomic emission spectrometry.