Abstract: According to the present invention there is provided a sensitive quantitative assay for the determination of a hapten "X" which comprises: preparing a conjugate of the hapten X which conjugate is adapted to ellicit antibody formation, and injecting a mammal with said conjugate, resulting in a specific antibody against the said hapten, "anti-X"; preparing a conjugate of the hapten X and of another entity Y, which is either another hapten or a larger molecule, injecting a mammal with the entity Y if it is a larger molecule, or with a conjugate thereof if it is a small molecule which does not by itself ellicit antibody formation, so as to form anti-Y antibodies, preparing a conjugate of a suitable enzyme and the antibody against Y, i.e. an (enzyme)-(anti-Y)-conjugate; adsorbing the antibodies against X, i.e.