In this paper, the effects of interference on the performance of dual-hop transmission with fixed-gain amplify-and-forward relaying is presented in terms of bit-error rate (BER). Particularly, a scenario with multiple relays and multiple antennas at destination is considered jointly with optimum combining (OC). The moment-generating function (MGF) of the signal-to-interference-plus-noise ratio (SINR) at destination is derived in order to evaluate the average BER. The analytical results are validated using Monte-Carlo simulations for several situations, showing that the proposed scenario totally mitigates the interference while maintains a diversity gain of K, where K is the number of cooperative relays.