Pregnancy serum contains the beta-core fragment of human choriogonadotropin.

We have developed a time-resolved immunofluorometric assay (IFMA) for quantification of the core fragment of the beta-subunit (c beta hCG) of hCG. The assay uses two monoclonal antibodies. One antibody is immobilized onto the wall of a microtiter strip well, and the other one is labeled with a europium chelate. The assay is sensitive (0.44 pmol/L = 4.6 ng/L), but cross-reaction with free beta-subunits of hCG (beta hCG) prevents direct determination of c beta hCG in serum. To circumvent this limitation we separated hCG and beta hCG from c beta hCG by gel chromatography and quantified each component in the fractions by specific IFMAs. The high sensitivity of the newly developed IFMA enabled us to demonstrate that serum from pregnant women contains c beta hCG and elutes at the same position in gel chromatography as c beta hCG purified from urine. The proportion of c beta hCG to hCG in pregnancy serum was 0.012-0.045% (mean +/- SD, 0.028 +/- 0.01) on a molar basis. Our finding of c beta hCG in serum confirms earlier reports suggesting that proteolytic degradation of beta hCG in the kidneys may not be the only pathway by which c beta hCG is formed.