The non-radiative resonance energy transfer between purple membrane protein(Bacteruiorhodopsin) PM and ionophore A23187 was observed. The efficiency ofenergy transfer increases with the increasing of the concentration of A23187. Theincreasing of fluorescence intensity of A23187 in the system of PM-A23187 is mainly due to the energy transfer from PM to A23187, and partly due to absorbt-ion of the excitation wavelength by A23187. The effects of Ca2+ and Tb3+ on the energy transfer between PM and A23187 are not obvious. Both cations decrease the fluorescence of A23187 in the PM-A23187 system. The...