Commercial production of human insulin can be effected via transgenic expression in plant seeds. Thus, levels of insulin accumulation exceeding 0.1% of total cellular protein can be achieved recombinantly, through expression of the insulin with a single-chain antibody as a fusion partner. Production in seeds offers flexibility in storage and shipment of insulin as a raw material, and insulin retains its activity upon extraction from stored seed. Further, the amount of biomass subjected to extraction is limited, due to the relatively low water content of plant seeds.