We have used transgenic tobacco seeds to produce large amounts of a functionally active engineered antibody. A gene fusion encoding an antigen-binding single chain Fv protein (scFv) that recognizes the hapten oxazolone was constructed and used as a model. After characterization in a bacterial expression system, the scFv gene was cloned into a plant expression cassette conferring seed specific expression, and transferred using Agrobacterium-mediated transformation, into Nicotiana tabacum. The expressed scFv could be detected in developing as well as ripe seeds of regenerated transgenic plants, and the functionally active scFv is stably deposited and accumulates up to 0.67% of the total soluble seed protein. After storage of ripe transgenic tobacco seeds for one year at room temperature there was no loss of scFv protein or its antigen-binding activity.