We propose a differential modulation scheme for
a two-user MIMO system based on orthogonal space-time block codes when neither
the
transmitter nor the receiver knows the channel. We derive low complexity
differential 
decoders for users with two transmit antennas. Moreover, we analyze the
diversity of the proposed
schemes and present simulation results. To the best of our knowledge, this is
the first 
low complexity differential scheme for multi-user MIMO systems.