Expression of foreign proteins in plants has become a standard technique in plant molecular biology. Various plant species have been used to produce mammalian proteins, such as human interferon (1) and serum albumin (2), as well as murine antibodies. Not only full-size antibodies (3–6) but also Fab fragments (7) and single-chain fragments (scFvs) (8,9) have been expressed successfully in tobacco orArabidopsis, reaching expression levels as high as 1.3% of the total soluble protein (3). ScFvs have also been expressed in plant-suspension cultures at levels of 0.5% total soluble protein (10). The feasibility of expressing and targeting recombinant antibodies (rAbs) (11,12) has been achieved in different compartments of plants, including the cytoplasm (scFvs), endoplasmic reticulum, chloroplasts, and the intercellular space (full-size, scFvs, and single-domain antibodies) for various applications (3–10). These results indicate the flexibility of the plant system for expression of rAbs, or fragments thereof, in various plant cell compartments.