Terbium (Tb^'*") was ušed as a fluorescence probe in the study of calcium-binding sites on 33 kDa protein of photosystem 2. The fluorescence of Tb^^ was enhanced markedly when bound to the 33 kDa protein, and the non-radiative energy transfer between tryptophan (Trp) residue and Tb^+, bound to the calcium-binding sites on the 33 kDa protein, took plače. According to the Forster non-radiative energy transfer mechanism, the average distance between the bound Tb3+ and Trp residue was found to be 1.05 nm. The pH titration indicated that major groups in the 33 kDa protein, involved in Ca2+ ions binding, were the carboxylic side groups of the glutamic acid and/or aspartic acid.