Plant-based expression of recombinant proteins offers significant advantages over transgenic animal-and cell-based systems. Unlike bacteria, plants perform the complex protein-processing steps required to produce eukaryotic proteins in active form. In order to facilitate protein production and purification we used hairy root cultures as a secretion-based in vitro plant system. We utilized the green fluorescent protein (GFP) as our model protein and expressed it for secretion in tobacco hairy root cultures. For large scale production of GFP, we adapted the Life-Reactor trade mark plastic sleeve bioreactor for growth of hairy roots in cultures containing up to 5 L of medium. Yields higher than 800 micro g of GFP per liter of culture were obtained after 21 d of incubation, representing almost 20% of the total secreted protein. The use of the plastic sleeve bioreactor system for expression of proteins in hairy roots allows for continuous or inducible production and recovery, while maintaining absolute containment, of the recombinant product.