BSA is unfolded by the surfactant CPB and is complexed with Tb3+ (Tb-BSA-CPB). At the excitation wavelength of 290 nm, both BSA and CPB get excited. The CPB emission, which is comparatively blue shifted (310 nm) than that of BSA (340 nm), overlaps with the absorption band of BSA resulting in the energy transfer from CPB to BSA. BSA transfers its energy to Tb3+ which emits at 490 and 545 nm. When Gd3+ is added to this system, the fluorescence of Tb3+ is found to be enhanced significantly. This is explained with regard to the intermolecular energy transfer from BSA of Gd-BSA-CPB species to Tb3+ of Tb-BSA-CPB species and to the energy-insulating sheath by Gd-BSA-CPB complex around Tb-BSA-CPB complex. This greatly enhanced the fluorescence of Tb3+ under optimal concentrations of CPB and Gd3+ is linearly correlated to the BSA concentration with ng/mL sensitivity, thus providing a sensitive determination of the protein concentration. (C) 2004 Elseviert B.V. All rights reserved.