The secretion of a functional, full-length monoclonal antibody complex from transgenic Nicotiana tabacum roots has been demonstrated. Initially, seeds were germinated on nitrocellulose membranes and antibody secretion detected from the developing roots. Plants were then established in hydroponic culture and secretion into the growth medium measured over 25 days. Western blotting indicated that full-length antibody was present in the medium along with other fragments. Secreted antibody was shown to be functional by binding to antigen in ELISA studies. In contrast, no antibody could be detected from transgenic Nicotiana in which the same antibody was expressed as a membrane protein in the plasmalemma. These results indicate that antibody accumulation in the growth medium is genuinely caused by rhizosecretion and not cell damage. Addition of gelatin to plant growth medium markedly increased levels of antibody accumulation. The mean antibody yield per plant was calculated to be 11.7 microg per gram root dry weight per day. Rhizosecretion may be a viable alternative to agricultural production or cell culture for the generation of monoclonal antibodies in transgenic plants. It may also give rise to novel applications for antibodies expressed in plants such as removal or neutralisation of environmental pollutants and attenuation of pathogens which infect the plant via the rhizosphere.