Healthcare providers are challenged by the availability of information at the point of care. Overcoming these challenges requires access to current, reliable, and accurate information. The purpose of the PubMed on Tap project is to discover and implement design principles to facilitate healthcare practitioners' access to medical information at the point-of-care, related to user interface design, organization and performance. The PubMed on Tap prototype was developed for personal digital assistants (PDAs) as a testbed to study these design principles. Usability testing was used to derive conclusions regarding the functionality of PubMed on Tap. Participant events (n= 323) mostly occurred in navigational ability (31%) and perceivedfunctionality (33%). A strong correlation (r= 0.87) was observed between total time and the sum of negative events in locating functions and navigational categories. Researchers concluded the basic functionality of the PubMed on Tap interface was valuable to users. However, there was room for improvement in the navigational and functional characteristics, such as placing labels on icons, which affected the users' ability to use the research tool.