We have shown that it is possible to transmit bovine spongiform encephalopathy (BSE) to a sheep by transfusion with whole blood taken from another sheep during the symptom.free phase of an experimental BSE infection. BSE and variant Creutzfeldt-Jakob disease (vCJD) in human beings are caused by the same infectious agent, and the sheep-BSE experimental model has a similar pathogenesis to that of human vCJD. Although UK blood transfusions are leucodepleted—a possible protective measure against any risk from blood transmission—this report suggests that blood donated by symptom—free vCJD-infected human beings may represent a risk of spread of vCJD infection among the human population of the UK.