Stirred-tank reactors (STR), in which gas and liquid phases make intimate contact for mass transfer, are quite common in chemical processes. Mechanical agitation enhances the mixing performance and the oxygen transfer capability in bio-STR, but they cannot be aerated at high rates because of impeller flooding. Another type of reactor widely used in bioprocessing is the airlift bioreactor, which consists of a draft tube and operates with an internal airlift loop. In this work, the mass transfer characteristics of a draft tube-impeller agitation system (combination of a conventional airlift and a stirred bioreactor) were investigated in a large-scale tank. The draft tube design configuration shows superior mass transfer performance compared to a conventional down-pumping agitation system consisting of a Rushton turbine combined with a pitched-blade turbine.