Traditionally, journalists, government agencies, and medical professionals have acted as mediators, facilitating the transfer of scientific knowledge from scientists to the general public. More recently, however, ordinary citizens are circumventing top-down mediation and contributing directly to discussions about scientific topics online. For the present study, we examined how these emerging mediators of online scientific information are shaping the discussion of hotly debated (at least within the public sphere) scientific topics, specifically, the alleged link between autism and the measles, mumps, and rubella (MMR) vaccine. Using content analysis, we have identified the resources that lay pro- and anti-vaccination knowledge contributors most often cite when making knowledge claims. Additionally, we examined how these contributors 1) use citations to legitimize their arguments; and, 2) take on particular roles in such arguments. Our results shed light on an emerging form of online science communication and the process by which knowledge contributed by ordinary citizens is shaping these online discussions. These findings have implications for online health information and health decision-making.