Green plants were an important historical source of medicines, and a primary source of humans’ first biopharmaceuticals. Because plants are the most efficient producers of protein on the planet, they are now being tapped as a production source for the fastest growing class of new pharmaceuticals: therapeutic and prophylactic proteins. The list of potential protein-based pharmaceuticals includes structural proteins from viral and bacterial pathogens, human antibodies and other immunoglobulins, defensins, enzymes and unique synthetic or chimeric proteins. Because plants are eucaryotes with a well developed endomembrane system, they have proved particularly efficient in producing complex and multimeric proteins. For complex proteins like immunoglobulins, green plants provide breakthroughs in economics, scale of production and the scope of molecules that can be efficiently manufactured. Producing antibodies in large quantities using professional agronomic corps could dramatically increase the use of antibody therapeutics and facilitate new uses for these medicines. While significant improvements are on the horizon, the basic technology for generating transgenic plants that produce a variety of functional antibodies is now well developed. The current challenge is to integrate this robust technology into the existing regulatory framework for developing safe and effective pharmaceuticals. We are witnessing the evolution of crop stewardship, manufacturing and processing technologies that protect health and safety of the public while delivering cost-effective treatments and preventives for human disease.