Preparation and Characterization of Antibodies to the Urinary Fragment of the Human Chorionic Gonadotropin β-Subunit*

hCG is a glycoprotein hormone composed of two dissimilar subunits (α and β) and is normally synthesized by trophoblastic tissue. Its measurement by immunoassay is widely employed as a test for pregnancy, but can be complicated by cross-reactivity with human (h) LH. Immunoassays based on the β-subunit of hCG have been employed to decrease this crossreactivity with hLH, but when these assays are used with urine specimens, the antibodies employed also detect a fragment of hCGβ, which can lead to significant differences in measurement. To overcome these problems, we have developed a series of monoclonal antibodies to the β fragment of hCG recovered from pregnancy urine. Some of the antibodies that bind to this β fragment are directed to a region of hCGβ that is different from the epitopes recognized by antibodies raised against the intact β-subunit. The new epitopes available in the hCGβ fragment form the basis for novel immunoassays. These β fragment antibodies are used in conjunction with other antibodies, directed to different epitopes of the hormone, to produce a series of immunoradiometric assays that can discriminate among intact hormone, free hCGβ, and hCGβ fragment. The hCGβ fragment antibodies described herein have affinities between 109 and 1011 M-1 for the β fragment and exhibit varying degrees of discrimination between the hCG/8 fragment, the β-subunits of hCG and hLH, and intact hCG and hLH. (Endocrinology123: 584–593, 1988)