Biological reactions are mostly concerned with selective interactions between small ligands and macromolecular receptors. The same ligand may activate responses of different intensity and/or effects in the presence of different receptors. Several experimental and theoretical approaches have been developed to study the recognition processes between ligands and receptors, including methods based on Nuclear Magnetic Resonance (NMR) analysis of the solution behaviour of the ligand in the presence of receptors. In this article we review one approach to the study of ligand-macromolecule interactions. These studies are based on: i) comparison of selective (R₁^SE) and non-selective (R₁^NS) spin-lattice relaxation rate of the ligand in the presence and absence of macromolecular receptors and ii) (R₁^NS) and (R₁^SE) temperature dependence analysis. From these studies, the ligand-receptor binding strength was defined on the basis of the «affinity index»,  [A]_L^T. The derivation of the «affinity index» from chemical equilibrium kinetics and its application to several problems will be widely discussed.