Human chorionic gonadotropin (hCG), a glycoprotein hormone composed of two nonidentical α- and β-subunits, is normally produced by trophoblasts. Serum and urine from some patients with nontrophoblastic tumors are found to contain similar immunoactivity to the β-subunit of hCG and this phenomenon has been recognized as an ectopic hCGβ production (1). Elevated levels of ectopic hCGβ are detected more frequently in urine than in serum. Recent studies have shown that urinary ectopic hCGβ mainly represents hCGβ-core fragment (β-CF) (2). Urinary β-CF consists of residues 6-40 disulfide bridged to residues 55-92 of the β-subunit of hCG (3,4), suggesting that it may be a degradative product of hCGβ. Such partial identity of the amino acid sequence between intact hCG, free hCGβ, and β-CF makes their specific measurements difficult. The existence of two more antigenic sites unique to hCGβ has been demonstrated. The one domain locates in the β-core portion and the other in COOH- terminal region of the β-subunit of hCG Fig. 1). By selecting appropriate pairs of antibodies to construct sandwich enzyme immunoassays (EIAs), it is possible to design methods that measure either intact hCG, free hCGβ, or β-CF Table 1). By using this EIA, the authors assessed the clinical usefulness of urinary β-CF as a tumor marker for nontrophoblastic tumors (5,6). Here, the authors describe the methods to measure urinary β-CF in patients with ovarian cancer. Fig. 1. Two antigenic sites: the first domain is located in the β-core portion and the second domain is located in the COOH-terminal region of the β-subunit of hCG. Table 1 Specificities and Sensitivities of Enzyme-Immunoassays Specific for Intact hCG(EIA-1), Free hCGβ(EIA-2), and β-CF(EIA-3) First antibody Second antibody Cross-reactivity (%) Sensitivity Intact Free Assay Code Epitope Code Epitope Specificity (ng/ml) hCG hCGβ β-CF EIA-1 No.277(MoAb) β-CTP No.224(MoAb) hcGa Intact hCG 0.01 100 15 1.0 EIA-2 No.209(MoAb) hCGβ No.115(PoAb) β-core Free hCGβ 0.01 1.9 100 1.9 EIA-3 No.229(MoAb) β-core No.105(PoAb) β-core β-CF 0.01 3.3 100 100 MoAb: monoclonal antibody. PoAb: polyclonal antibody.