In recent years, the selective flow of knowledge has emerged as an important topic in historical and social studies of science. Related questions about the production of ignorance have also captured attention under the rubric of agnotology. This paper focuses on information control in interaction, examining how actors seek to control the flow of scientific knowledge as they interact with others, either in face-to-face encounters or in modes of communication involving circulating documents, data, materials and other entities containing knowledge. The analysis uses an ethnographic approach to study how actors work to control which knowledge becomes available to whom, when, under what terms and conditions, and with what residual encumbrances. Secrecy, for example, is not framed as an isolated, sui generis phenomenon, nor as one side of a secrecy/openness dichotomy, nor even as a pole on a secrecy/openness continuum. Instead, the analysis explores how actors manage a dialectic of revelation and concealment through which knowledge is selectively made available and unavailable to others, often in the same act. The emphasis on selective revelation highlights partial transfers of knowledge, targeted distribution, matters of timing, and the rights and encumbrances that attach to knowledge at different points in its transit. Examples are drawn from genome research, a field marked by ongoing disputes about modes of information control.