The modulation of the currents in a symmetric Semiconductor-on-Insulator (SOI) lateral bipolar transistor with a voltage applied to the SOI substrate is studied. For an n-p-n transistor, a positive substrate bias could greatly increase the collector current, especially at low values, while having relatively little effect on the base current. Similarly, a negative substrate bias could greatly increase the collector current of a p-n-p transistor. The physical mechanisms responsible for the modulation effects are discussed. The potential of using substrate bias to enhance the performance of symmetric SOI lateral bipolar circuits is briefly discussed.