Thirty-nine monoclonal antibody (MCA) producing hybridoma cell lines derived from fusions of mouse myeloma cells with spleen cells from mice immunized with human chorionic gonadotropin (hCG) have been established. Their products have been tested in radioimmunoassays using 125I-labeled hCG, luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), the alpha (alpha) and beta (beta) subunits of hCG and LH, and the C-terminal peptide 109-145 (CTP) of CG. All MCA were, in addition, tested in indirect immunofluorescence (IIF) on paraffin sections of human pituitary glands. According to the intramolecular localization of the determinants recognized, three main groups of MCA can be distinguished: 1) MCA directed against epitopes on the alpha-chain(alpha-MCA), 2) MCA directed against beta-chain determinants(beta-MCA), and 3) MCA reacting with a conformational determinant only present on the native hormone and not on either subunit (conformational-MCA). All alpha-MCA cross-react with human LH, FSH, and TSH. The beta-MCA do not react with FSH or TSH, but do react to a varying degree with LH. The conformational-MCA show no binding of labeled FSH or TSH and very little or no cross-reactivity with LH. (Am J Reprod Immunol. 1982; 2:212-216.)