An 'affinity index', representing the global affinity between a ligand and a biomacromolecular receptor, is proposed. It is determined as the slope of the linear relations between Delta R(1)(SE) and the receptor concentration and has the advantage of providing a measure of the ligand-biomacromolecule global affinity which is independent of the number of interaction sites. The method was applied to the calculation of the lamotrigine-albumin affinity index.