This paper presents an extended case study to demonstrate that the interpretation of clinical trials of antiviral AIDS drugs is significantly shaped by a widely dispersed allocation of scientific credibility. Specifically, the participation of AIDS activists in claims-making about AIDS trials and AIDS drugs complicates the politics of therapeutic evaluation, even as it challenges the monopolization of credibility by credentialed researchers. The paper tracks the social construction of belief about the efficacy of the combination therapy of AZT and ddC, between 1990 and 1995 in the United States. By intervening simultaneously in interpretative debates about the results of the clinical trials of this therapy and in methodological debates about how efficacy might best be measured in such trials, activists have helped to shape what is believed to be known about these drugs.