As the search for human genetic variation has become a priority for biomedical science, debates have resurfaced about the use of race and ethnicity as scientific classifications. In this paper we consider the relationship between race, ethnicity and genetics, using insights from science and technology studies (STS) about processes of classification and standardization. We examine how leading biomedical science journals attempted to standardize the classifications of race and ethnicity, and analyse how a sample of UK genetic scientists used the concepts in their research. Our content analysis of 11 editorials and related guidelines reveals variations in the guidance on offer, and it appears that there has been a shift from defining the concepts to prescribing methodological processes for classification. In qualitative interviews with 17 scientists, the majority reported that they had adopted socio-political classification schemes from state bureaucracy (for example, the UK Census) for practical reasons, although some scientists used alternative classifications that they justified on apparently methodological grounds. The different responses evident in the editorials and interviews can be understood as reflecting the balance of flexibility and stability that motivate standardization processes. We argue that, although a genetic concept of race and ethnicity is unlikely to wholly supplant a socio-political one, the adoption of census classifications into biomedical research is an alignment of state bureaucracy and science that could have significant consequences.