The gene encoding enterotoxigenic Escherichia coli B-subunit heat-labile toxin (LTB) antigen was co-transformed into hairy root cultures of Nicotiana tabacum (tobacco), Solanum lycopersicum (tomato) and Petunia parodii (petunia) under the CaMV35S promoter. Tobacco and petunia roots contained similar to 65-70 mu g LTB g(-1) tissue whilst hairy roots of tomato contained similar to 10 mu g LTB g(-1). Antigen at similar to 600 ng ml(-1) was detected in growth medium of tobacco and petunia. Tobacco roots with higher LTB levels showed growth retardation of similar to 80% whereas petunia hairy roots with similar levels of LTB showed only similar to 35% growth retardation, relative to vector controls. Regeneration of plants from LTB-containing tobacco hairy roots was readily achieved and re-initiated hairy roots from greenhouse-grown plants showed similar growth and LTB production characteristics as the original hairy root cultures.