Abstract: A method is described by which the association between an oligonucleotide labeled by attachment of a fluorophore and another macromolecule such as a protein or nucleic acid may be determined quantitatively in solution accurately and with high sensitivity. In the performance of this method the polarization of fluorescence of an extrinsic fluorescence probe that is covalently coupled to the oligonucleotide is determined. Changes in fluorescence polarization are related directly to the degree of association between the labeled oligonucleotide and another macromolecule and may be used to quantify the association. Because of its high sensitivity and accuracy, this method may be used to make reliable quantitative measurements of very small amounts of complexes formed between labeled oligonucleotides and proteins, nucleic acids or other macromolecules. The method also allows the accurate calculation of important biochemical parameters such as dissociation constants.