Clinical medicine and biotechnology increasingly utilise and transform human bodily tissues in novel ways. Today more and more tissues--blood, whole organs, ova, embryos, sperm, skin, bone, heart valves, cellular material, bone marrow and corneas--can be transferred between donors and recipients. Hence more and more people in developed nations have the experience of giving a fragment of their body to another, or receiving such a fragment as part of some kind of therapy. These systems for the circulation of tissues raise the question of what we have termed 'bioidentity'. Bioidentity describes our common-sense understanding of our bodies as 'ours', as both supporting and being included in our social and subjective identities. Within this framework, how are we to understand the status of detachable bodily fragments like blood, ova or organs? As parts of our bodies do they retain a trace of our identity after donation, or are they detachable things? What is our relationship, if any, to the patient who receives our tissues as part of their treatment? This paper investigates the specific case of blood transfusion and donation. It draws upon in depth interviews with 55 people who have specific experience with blood. They either have hepatitis C (are HCV+) acquired by transfusion or intravenous drug use, or have donated blood or received a blood transfusion but are free of hepatitis C (are HCV-). We analyse this material according to the themes--Donated Blood as 'Self', Blood as Alienable, Blood as Communal Substance, and Contaminated Gifts and the Blood of Strangers. We find that, generally speaking the HCV+ and HCV- groups share very similar ideas about blood donation and transfusion. For a minority of both groups, blood was understood as a decisive site of self irrespective of location, but for the remainder donated blood was either ambiguous with regard to identity, a shared substance, or not considered to have any lingering relationship to the self once given. However both groups regarded blood as strongly imbued with 'risk identity'. In particular the HCV+ interviewees regarded their blood as a dangerous personal attribute, one that they must be careful to withhold from circulation, whereas the blood donors felt obliged to donate their blood precisely because they considered it clean and risk free.