Amperometric sensors may be used in milk analysis but electrochemical interference from compounds other than the analyte is an on-going problem. A survey was made of the level of electrochemical activity (potential interference) in milk from a herd of dairy cows grazing on summer pasture. It was a ubiquitous feature of the aqueous phase of whey and de-proteinized milk over about 3 months. The nature of the interference was studied by differential pulse voltammetry and responses to ascorbic acid oxidase and uricase. The principal source of interference appeared to be uric acid.