Several synthetic peptides, modelled from a Ca 2 + -binding loop of the EF-hand family of proteins, were prepared containing cysteine residues. The peptide, GDKNADGFICFEEL, was labelled covalently at the cysteine residue (loop position 9) with iodoacetamidosalicylic acid. This novel conjugate is a metal-binding loop containing a salicylic acid side chain that could not only chelate Tb 3 + in conjunction with the other chelating groups in the sequence, but could also sensitize Tb 3 + luminescence. The loop had a high Tb 3 + affinity, with stoichiometric binding observed under experimental conditions. The luminescence from the Tb 3 + -peptide complex was more than 10-fold greater than the luminescence reported from a related peptide which contained Trp as the Tb 3 + donor at loop position 7. This peptide has significant potential for use in lanthanide-based time-resolved luminescence immunoassays.