In existing multiuser MIMO (MU-MIMO) MAC protocols, a multi-antenna node sends as many concurrent streams as possible once it wins the contention. Though such a scheme allows nodes to utilize the multiplex gain of a MIMO system, it however fails to leverage receive diversity gains provided by multiple receive antennas across nodes. We introduce Multiplex-Diversity Medium Access (MDMA), a MU-MIMO MAC protocol that achieves both the multiplex gain and the receive diversity gain at the same time. Instead of letting a node pair use all the available degrees of freedom, MDMA allows as many contending node pairs to communicate concurrently as possible and share all the degrees of freedom. It hence can exploit the antennas equipped on different receivers to further provide some of concurrent streams more receive diversity, without losing the achievable multiplex gain. We implement a prototype on software radios to demonstrate the throughput gain of MDMA.