This article examines the reasons for, and likely impact of, the decision by the US and other countries to permanently defer blood donors who have spent time in Britain or Europe, for fear they may transmit new variant Creutzfeldt-Jakob disease (vCJD), the human form of "mad cow disease". It begins by discussing how vCJD and blood transfusion are linked, and how these have been translated into policy. First, maintaining a safe and stable supply of blood entails not only maintaining the trust of recipients in the system, but also that of donors, who need to be assured that their blood will be welcomed and used. Often, the balance, once upset, is regained by sacrificing donors, but accompanying costs might also be high. Second, the article highlights the impact of various forms of globalization -of commerce, disease and travel, and immigration- on blood policies and public and policy attitudes. Third, it assesses the decision by the US to restrict blood donations from Europeans and travelers to combat such a pervasive risk. The conclusion discusses how donor deferral policies may be interpreted by the public in the light of earlier discussions, and raises issues for future research.