Five β-diketone derivative were studied for multiple labelling of protcis. The labclled proteins were characterized by absorption and fluorescence measuremcnts. It Was found that proteins labelled with chlorosulfonyl thenoyltrifluoroacetone (CTTA) were able to form highly fluorescent complexes with Eu3+ which exhibite4d prolonged fluorescence wheresas the Eu3+ complex of hydrolyxed CTTA exhibited almost no fluorescence, and so unreacted ligand gave no background signal in immmunooassays even if it was not removed from the labelled reagent. The cffect of labelling on the biological activity of albumin and polycolonal antibody was studied and it was also shown that the new probe could be used in time-resolved fluorescence immunoassays.