Abstract: An improvement in carrying out assaying methods involving biospecific affinity reactions in which there are used from two to four reactants, one of which reactants, reactant (I), being labelled with at least one analytically indicatable atom or group and being soluble in the aqueous liquid wherein the biospecific affinity reaction is carried out, said reactants forming by biospecific affinity reactions a conjugate in which labelled reactant (I) is incorporated, and in which methods the analytically indicatable atom or group is assayed in the conjugate and/or in the free labelled reactant (I), said improvement being characterized in(a) using as at least one of said reactants a reactant which is modified in such a way that conjugate and free labelled reactant (I) are asymmetrically partitioned in a system of two immiscible aqueous liquid phases, which system in addition to water comprises in dissolved form(i) at least two polymers or(ii) at least one polymer and at least one salt or(iii) at least one polymer an