This article addresses the routine use of race and ethnicity in the context of applied population genetics, which includes research in genetic epidemiology into the causes of common, complex diseases and in pharmacogenctics - the study of the genetic basis of how people respond to pharmaceuticals. It investigates how categories of "whiteness" are operationalized in everyday scientific practice. It reviews the critique of the presumed homogeneity, invisibility and "common-sense" status of whiteness in the epidemiological and clinical literature, and draws on qualitative interviews conducted with twenty-eight UK-based researchers working in the areas of genetic epidemiology and pharmacogenetics. The study explores how researchers justify the inclusion of white/Caucasians in studies and the exclusion of other ethnic groups, operationalize white/Caucasian as a category by which volunteers or patients are selected in and out of studies, and show how genetic evidence can also challenge researchers' assumptions and problematize their everyday use of categories such as white/Caucasian.