Multiple transmit and receive antennas can be used in wireless systems to acheive high data rate communications. Recently, space-time codes have been developed that utilize some of the capacity available. These codes are developed under the assumption that the transmitter has no knowledge about the channel. However, we consider the case when the transmitter has partial, but not perfect, knowledge about the channel and how toadjust the antenna weights at the transmitter. A criterion is derived that optimally determines the transmit antenna weights when well known block orthogonal space-time codes are used. Simulation results using two transmit antennas are included to demonstrate the performance.