Erythrocyte glutathione peroxidase (GSH-px) assays are now commonly used as an alternative to blood selenium concentrations in assessing the selenium status of production animals. This can be justified on the grounds the GSH-px is a selenium-containing enzyme and that its activity is highly correlated with blood selenium concentration. Furthermore, GSH-px assays can be adapted to most automated biochemical analysers currently being used in diagnostic laboratories and are not subject to the problems of sample contamination that commonly interfere with estimations of trace elements