The use of monoclonal antibodies directed against epitopes only present on the free bHCG subunit enables a specific and ultrasensitive assay of this subunit. Furthermore, the assay presents no cross-reactions with LH, TSH, FSH or native HCG. The ELSA-FbHCG assay is particularly wel-adapted to the diagnosis and monitoring of trophoblastic tumors, either of placental (moles, chorionic carcinoma, chorioepithelioma) or testicular origin (seminomatous germ cell tumors or non-seminomatous tumors having a trophoblastic cell component). The use of such an ultrasensitive method means early diagnosis of therapeutic escape and of metastatic spread.