Plant cell cultures have been used as expression hosts for recombinant proteins for over two decades. The quality of plant cell culture-produced proteins such as full-size monoclonal antibodies has been shown to be excellent in terms of protein folding and binding activity, but the productivity and yield fell short of what was achieved using mammalian cell culture, in which the key to gram-per-liter expression levels was strain selection and medium/process optimization. We carried out an extensive media analysis and optimization for the production of the full-size human anti-HIV antibody 2G12 in N. tabacum cv. BY-2. Nitrogen source and availability was found to be one key factor for the volumetric productivity of plant cell cultures. Increased amounts of nitrate in the culture medium had a dramatic impact on protein yields, resulting in a 10–20-fold increase in product accumulation through a combination of enhanced secretion and higher stability. The results were scalable from shake flasks to stirred-tank bioreactors, where the maximum yield per cultivation volume was 8 mg L−1 over 7 days. During the stationary phase, antibody levels were 150-fold higher in nitrogen-enriched medium compared to standard medium. The enhanced medium appeared not to affect antibody quality and activity, as determined by Western blots, surface plasmon resonance binding assays and N-glycan analysis. Biotechnol. Bioeng. 2010;107: 278–289. © 2010 Wiley Periodicals, Inc.