The oxidase activity of caeruloplasmin towards Fe II (ferroxidase) appears to be of physiological significance both in iron metabolism and as a serum anti-oxidant. We have developed an automated assay for ferroxidase using a centrifugal analyser. The method is quick and precise, and is not subject to interference by haemolysis, jaundice or lipaemia. Ferroxidase activities correlate significantly with caeruloplasmin and copper concentrations. Normal reference ranges have been measured.