Abstract: A two-site sandwich immunoradiometric assay for quantifying CA15-3 in human serum is established by using two monoclonal antibodies. One of the antibodies is labeled with ~(125)I as a tracer and the other recognizing a different antigenic determinant of CA15-3 antigen is immobilized on poly-styrene tubes as the solid phase. The standards prepared from CA15-3 antigen are corrected against the standards of CIS ELSA CA15-3 IRMA kit. The sensitivity of the assay is 0.4 U/mL and the recovery is 86.8%～ 118.6%. The intra- and inter-assay CVs are 6.0%～6.2 % and 4.1%～5.9%, respectively. The correlation coefficients between measured and expected values are more than 0.99 after serial dilution of the samples with high concentrations of CA15-3. The value for female normal samples (n=38) is 5.7～23.7 U/mL.