This paper begins with a remarkable moment in contemporary Malaysian politics when a contested blood sample of the leader of the opposition was claimed by some as having the capacity to ‘reveal the truth’ about his character. What is it about blood that gives it this iconic status? Drawing on research carried out in hospital clinical pathology labs and blood banks in Penang, the paper shows how blood samples, far from being detached from persons, may accrue layers of meaning as they travel round the lab. This occurs partly through the special properties of blood, and partly through the socially embedded interventions of laboratory staff. Tracing the social life of blood also allows us to grasp how the separation between domains of social life, which is fundamental to an ideology of modernity, must be laboriously achieved, and can often be only precariously maintained. In the case of blood, however, the stakes may be unusually high when the boundaries between, for example, biomedicine and politics or kinship become over-permeable or threaten to collapse.