Study and application of europium (Eu) labeled mouse monoclonal antibody (McAb) C₁₇ against carcinoembryonic antigen (CEA) by using a synthesized chelate-N¹-(p-isothiocyanatobenzyl)-diethylenetriamine-N¹,N²,N³, N³-tetraacetic acid Eu is described.The Eu-labeled C₁₇ was separated from excess reagent by gel filtration on a column of Sephadex G-50.The fluorescence and ultraviolet absorption for the fractions were monitored and the curves of fluorescence and the optical density were all the same.The specific activity for the two tubes of the labeled-C₁₇ at the protein peak was 5.2 and 9.8 Eu³⁺/C₁₇ molecule respectively and the recovery of the labeled-C₁₇ was 64%. The labeled-C₁₇ was stable for 6 months at least. The standard curves of time-resolved immunofluorometric assay using the labled-C₁₇ showed excellent linearity and slope and had been used in the assay of CEA in serum.