Reversed-phase ion-pair chromatography was used to monitor the oxidation of four biogenic amines (adrenaline, noradrenaline, dopamine and 5-hydroxytryptamine) by the copper-containing protein, caeruloplasmin. The methods are reproducible and sufficiently rapid to permit the handling of plasma sample batches. Kinetic parameters obtained using the method are in good agreement with those obtained by more traditional means of enzyme assay. Finally, the results support the view that more than one site on the enzyme, whether binding or oxidative, may be involved in the oxidation of biogenic amines, and the possible implications of this are briefly discussed.