Abstract: The invention relates to a detectable molecule comprising a biospecific binding reactant attached to a luminescent lanthanide chelate comprising a lanthanide ion and a chelating ligand of the formula (I) wherein, R1A R1B are independently of each other selected from the group consisting of hydrogen, methyl, ethyl, —COOH, —COO—, —CH2COOH, —CH2COO—, hydroxyl or OR2; R2 is selected from the group consisting of —CH3, —C(CH3)3, —C(CR4)3, wherein R4 is an alkyl with 1 to 6 carbon atoms, —CH2COOH, —CH2COO?, and appropriate hexose residues; R3 is a linker for coupling to a biospecific binding reactant selected from the group consisting of thiourea (—NH—CS—NH—), aminoacetamide (—NH—CO—CH2—NH—), amide (—NH—CO— and —CO—NH—), aliphatic thioether (—S—), disulfide (—S—S—) and 6-substituted-1,3,5-triazine-2,4-diamine; and the lanthanide ion is selected from the group consisting of europium(III), terbium(III), dysprosium(III) and samarium(III). The invention also relates to corresponding lanthanide chelates.