Abstract: A method for indirect measurement of the concentration of a free ligand in a liquid sample also containing the ligand bound to endogenous binding material, comprising the steps of performing two separate measurements, wherein the first measurement is used to determine the total concentration of free and reversibly bound ligand in the sample to be tested, and the second measurement comprises the steps of (i) exposing separately to a known amount of a labelled ligand (a) the sample to be tested, and (b) each of a plurality of standard solutions containing known concentrations of the free ligand, (ii) thereafter exposing each resulting mixture from (i) (a) and (i) (b) separately to an immobilized binding agent to bind a portion of the free labelled ligand and a similar proportion of the free unlabelled ligand to the immobilized binding agent in each mixture, (iii) separating the immobilized binding agent with its bound ligand from the residual material in each mixture, (iv) measuring the proportion of labelled l