Abstract: A twosite sandwich immunoradiometric assay for quantifying CA125 in serum is developed 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 the CA125 molecule is immobilized to plastic tubes as the solid phase. The standards are prepared from the abdominal fluid of ovarian cancer patients. The ratio of the maxium dose binding and nonspecific binding of the standard curve is more than 100.The sensitivity of the assay is 1.0 U/mL and the recovery of added CA125 is 96.8%~112.5%. The intra and interassay CVs are 4.4%~5.8% and 8.4%~9.9% respectively. The correlation coefficients between measured and expected values are more than 0.99 after dilution of the samples with high concentrations of CA125. The value for female normal samples(n=38) is 2.9~24.1 U/mL.