Transgenic plant cell cultures offer a number of advantages over alternative host expression systems, but so far relatively low product concentrations have been achieved. In this study, transgenic rice cells are used in a two-compartment membrane bioreactor (CELLine 350, Integra Biosciences) for the production of recombinant α-1-antitrypsin (rAAT). Expression of rAAT is controlled by the rice α-amylase (RAmy3D) promoter, which is induced in the absence of sugar. The extracellular product is retained in the bioreactor's relatively small cell compartment, thereby increasing product concentration. Due to the packed nature of the cell aggregates in the cell compartment, a clarified product solution can be withdrawn from the bioreactor. Active rAAT reached levels of 100-247 mg/L (4-10% of the total extracellular protein) in the cell compartment at 5-6 days postinduction, and multiple inductions of the RAmy3D promoter were. demonstrated.