Drawing on a UK research study on immunization, this article investigates parents' understandings of the relationship between themselves, their infants, other bodies, the state, and cultural practices--material and symbolic. The article argues that infant bodies are best thought of as always social bundles, rather than as biobundles made social through state intervention; and concludes that, while the natural/cultural divide may now be widely accepted as artificial within the social sciences, we need to scrutinize "how" people in their everyday lives work out, and invest in, the distinction between the two. (Contains 12 notes.)