This invention describes a process for immuno-chemical quantitative determination of immunologically active substances. The method involves a first incubation of a sample containing the active substance with a labelled binder, which contains an antibody or antibody fragment. After complexing has taken place, solid phase bound active substance identical to the substance being quantitatively determined is added. The solid phase bound substance binds with free binder, and the solid and liquid phases are separated. A second antibody which is specific either to antibody or the solid phase antibody-substance complex is then added to the liquid phase. This second antibody is non cross-reactive with individual complex components. The amount of labelled first antibody bound to second antibody is then determined.