Three plant transformation vectors (pBI?CTB, pBI?SPCTB and pBI?CTBER) were constructed to express CTB under the control of the CaMV 35S promoter. Tobacco was transformed by co?cultivating leaf discs with Agrobacterium strains harboring pBI?CTB, pBI?SPCTB and pBI?CTBER respectively. The regenerated kanamycin?resistant transformants were analyzed by PCR, Southern blot, ELISA and Western blot. These results indicated the CTB gene integrated in the tobacco genomic DNA and was expressed efficiently. CTB levels i...