The binding of Ca2+ ions to bovine and human thyroglobulin (Tg) was demonstrated qualitatively by 45Ca overlay on polyvinylidene difluoride (PVDF) membranes. A quantitative analysis of the interaction of metal ions with bovine Tg was conducted by fluorimetric titration of the protein with Tb3+ ions. These have been used with several proteins as isomorphous replacement probes for Ca2+ ions, as protein-bound Tb3+ ions fluoresce, upon irradiation in the UV region, because of energy transfer from tyrosyl and/or tryptophanyl residues. The fluorescence emission spectrum of Tg excited at 280 nm showed, upon addition of Tb3+ ions, a peak at 546 nm and a marked decrease at 335 nm, indicating an efficient Förster energy transfer between bound Tb3+ ions and closely located Tg intrinsic chromophores. Titration of Tg with Tb3+ ions, carried out by monitoring the emitted fluorescence at 546 nm, indicated the presence of 13.15 metal binding sites per Tg molecule.