The reporter gene β-glucuronidase was transiently expressed in a 51-L bioreactor-grown plant cell suspension culture of Nicotiana glutinosa at a yield of approximately 1.1 mg through co-culture with an auxotrophic strain of Agrobacterium tumefaciens. The three order of magnitude scale-up involved the investigation of factors contributing to transient expression including the timing of Agrobacterium inoculation relative to the plant cell growth phase, plant tissue culture hormonal triggers and plant cell cycle synchronization. The co-culture process was simplified to facilitate implementation in a pilot-scale bioreactor. At the shake flask scale it was determined that elevated concentrations of oxygen in the headspace were detrimental to transient expression levels and the addition of acetosyringone to the co-culture had a negligible effect. The bacterial preparation process was also streamlined, permitting the direct transfer of the Agrobacterium culture from a bench-scale fermentor to the pilot-scale plant cell culture bioreactor. Increasing expression levels and overcoming batch-to-batch variability despite extensive procedure systemization remain the major technical hurdles.