Abstract: Disclosed is a method for the direct determination of the free fraction of analytes present in biological fluids in a free form and in a form bound to one or more endogenous ligands (said free and bound forms being in equilibrium with one another). This method provides for a (preferably substantially simultaneous) use:a first ligand L1 capable of sequestering an analyte quantity proportionate to the free-analyte concentration present in a biological fluid and to subsequently release it, after removal from the biological fluid of the specific endogenous ligand, as a result of the addition of an appropriate selective dissociating agent; a second ligand capable of binding both the previously released analyte and a labelled version of the analyte, even in the presence of the dissociating agent; a selective dissociating agent; and a quantity of labelled analyte.