The activity in vitro of the bacitracin synthetase of B. licheniformis ATCC 10716 is influenced by divalent metal ions (Mg2+, Mn2+, Fe2+ or Co2+) and by bacitracin. Complexes between bacitracin and metal ions exert feedback control on the synthetase. An overall control mechanism for bacitracin synthetase may consist of substrate-metal ion complexes and product-metal ion complexes.