A Cd2+-resistant Escherichia coli C600 transformant harboring pGU100, which was derived from Cd2+-resistant Pseudomonas putida GAM-1, was able to grow in concentrations of CdCl2 as high as 3.5 mM, whereas E. coli C600 could not grow in the presence of 1.5 mM CdCl2. E. coli C600 (pGU100) possesses a Cd2+ efflux system. This efflux system was inhibited by 100-mu-M dicyclohexylcarbodiimide, indicating that the system seems to be energy-dependent. Further studies revealed that the Cd2+ efflux system of E. coli C600 (pGU100) can operate under proliferous conditions, but not under nonproliferous conditions.