Fractionation of normal serum on Sephadex G-150, followed by determination of copper, caeruloplasmin and albumin concentrations, indicated that only approximately 71% of total serum copper was associated with caeruloplasmin; less than previously reported values. Seven per cent was associated with a high molecular weight protein, designated 'transcuprein', 19% with albumin and 2% with amino acids. Compared with adult serum the concentrations of caeruloplasmin and of copper associated with caeruloplasmin were low both in serum from neonates and in serum from patients with symptomatic Wilson's disease. However, in contrast to the neonate, Wilson's disease patients exhibited a raised total serum copper and raised non-caeruloplasmin-copper. In Indian Childhood Cirrhosis serum caeruloplasmin and caeruloplasmin-copper levels were normal, whilst the non-caeruloplasmin-copper was raised. Elevated non-caeruloplasmin-copper in Wilson's disease and Indian Childhood Cirrhosis may therefore represent an overspill into the serum from a copper-laden liver. Children with malignancy showed increased serum concentrations of copper and caeruloplasmin. Both caeruloplasmin-bound and non-caeruloplasmin-bound copper concentrations were elevated. It remains to be determined whether increased 'transcuprein'- and albumin-bound copper result from a sequestering of copper released from peripherally utilized caeruloplasmin, or are associated with increased rates of caeruloplasmin synthesis.