In an unusual instance of lay participation in biomedical research, U.S. AIDS treatment activists have constituted themselves as credible participants in the process of knowledge construction, thereby bringing about changes in the epistemic practices of biomedical research. This article examines the mechanisms or tactics by which these lay activists have constructed their credibility in the eyes of AIDS researchers and government officials. It considers the inwlications of such interventions for the conduct of medical research; examines some of the ironies, tensions, (...) and limitations in the process; and argues for the importance of studying social movements that engage with expert knowledge. (shrink)