Open access to the peer-reviewed primary research literature would greatly facilitate knowledge transfer between the creators and the users of the results of research and scholarship. Criteria are needed to assess the impact of recent initiatives, such as the Budapest Open Access Initiative. For example, how many open-access research journals exist within a given field, and what is the reputation of each one? And, how many openly-accessible institutional e-print archives have been created and how many are actually are being used by researchers and scholars? A simple approach to an assessment of the open-access portion of the medical literature is described, and some preliminary results are summarized. These preliminary results point to the need for incentives to foster the implementation of initiatives such as the Budapest Open Access Initiative. An example of an incentive model is proposed, where an agency or foundation that provides peer-reviewed grants-in-aid to researchers establishes an e-print archive. Only current grantees of the agency would be eligible to post reports about the results of research projects or programs that have been supported by the agency. Some advantages and implications of this particular model are outlined. It is suggested that incentive models of this kind are needed to increase the likelihood that open access to the primary medical research literature will soon reach a "tipping point" and move quickly toward wide acceptance.