Space-time spreading has been employed to exploit the spatial diversity in multiple-input multiple-output (MIMO) code-division multiple-access (DS-CDMA) systems. In the presence of multiuser interference, resulting from the cross-correlation between users' code sequences, the full system diversity cannot be achieved when using the conventional matched receiver. In this paper we investigate the performance of space-time spreading (STS) and transmit diversity in the uplink of a MIMO DS-CDMA system over Nakagami-m fast-fading channels. The space-time system employs N = 2 transmit antennas and L receive antennas at the user side and base-station, respectively. We analyze the performance of the system when a linear decorrelator detector is employed to mitigate the effect of multiuser interference. In our analysis, we start by finding the probability density function (pdf) of the signal-to-interference ratio (SINR) at the output of the space-time combiner. Using this pdf, we derive a closed form expression for the bit-error rate (BER) for binary phase-shift keying (BPSK) transmission. The accuracy of the derived pdf and BER expression are verified using simulations, for different values of fading figure m and number of users.