Abstract: The invention concerns a method of homogeneous immunoassay of a ligand in a liquid test sample which comprises: a)incubating a mixture of (1) the liquid test sample; (1i) where the ligand under assay has only one epitope, a covalent conjugate of the ligand with any mono- or poly-epitope molecule having at least one epitope distinct from the epitope of the ligand; (2) a labile enzyme; (3) a first bispecific monoclonal antibody protecting the activity of the encyme through binding to a first epitope on the encyme and capable of binding the ligand under assay; and (4) a second bispecific monoclonal antibody capable of bonding said enzyme and a second distinct epitope on the ligand under assay or an epitope of the said covalent conjugate; whereby a quaternary immunocomplex is formed; b) after incubation, subjecting the mixture to conditions whereby any enzyme not present in such a quaternary immunocomplex is inactivated; and c) determining the amount of detectable encyme, the determination being related to the am