This research empirically evaluated the use of mobile information and communication technology in a large-sized undergraduate class, where the effectiveness of multilearner participation and prompt learner-instructor interaction is often challenged. The authors analyzed the effectiveness of a so-called “lean” communication medium using hand-held mobile devices, whose brief text-based messages considerably limit the speed of information exchange. Adopting a social construction perspective of media richness theory and a reinforced approach to learning and practice, the authors conjectured that an interactive learning system built with wireless PDA devices can enhance individual practices and reinforce peer influences. Consequently, they expected better understanding and higher satisfaction among learners. A field experiment with 118 participants in the treatment and 114 participants in the control group supported their hypotheses. Their results suggested that richness of a “lean” medium could be increased in certain socially constructed conditions, thus extending existing notions of computer-aided instruction towards a techno-social learning model.