Two distinct transient expression approaches were compared with assess the impact of the viral suppressor p19 on a recombinant protein production performed in Nicotiana benthamiana suspension culture. A parental N. benthamiana cell line was transiently transformed with either an Agrobacterium containing a gene construct for a murine IgG1 (R514) or concurrently with two Agrobacteria containing R514 or p19. In addition, a stably transformed N. benthamiana cell line that constitutively expresses p19 was transformed with R514-containing Agrobacterium. The parental N. benthamiana cell line that had been co-cultivated with both p19 and R514 achieved the highest yield of IgG1 (1.06 mg IgG1/kg FW; 0.024% TSP) compared with that obtained without p19 (0.61 mg IgG1/kg FW; 0.014% TSP). The N. benthamiana cell line that had been stably transformed with p19 only reached 0.25 mg IgG1/kg FW (0.009% TSP) when co-cultured with R514-containing Agrobacterium. Dual agroinfiltration of N. benthamiana leaves with p19 and R514 was also performed to assess for Agrobacteria efficiencies and 147.7 mg IgG1/kg FW were obtained. Therefore, our results demonstrate that transient co-transformation of plant cell suspension culture with two transformation vectors is feasible and that the use of the viral suppressor of silencing p19 significantly raises the production of the protein of interest. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1534-1543, 2010