Blood supplies have become indexes of national security and the public good. While blood shortages can provoke anxiety, controversies continue to erupt in many countries over proper donor screening, especially with reference to HIV. This article sketches these dynamics in several global settings, focusing especially on activist efforts by gay men to reform exclusionary blood donor guidelines. The contours of the debate recall familiar conflicts between the putative demands of public health and the rights of individuals in the era of AIDS. However, if gay activists marshal a discourse of individual rights vis-a-vis forms of institutional exclusion, they also seek a broader shift in social and cultural understandings of gay identity. To capture this complex interplay of citizenship and sociality, risk and responsibility, the article introduces the notion of `vital publics' to refer to the peculiar associational form represented by blood supplies. Vital publics are kinds of embodied association elicited through the generalized exchange of body — in this case, blood. Hailed to `give life' by the jargon of the pervasive social marketing of varied blood service systems, activists seek to contribute to the life of the `vital public' that transfusion medicine calls into being.