Abstract: A method of determining the binding of an oligonucleotide probe to a test nucleic acid sequence comprises the steps of: (a) providing the test nucleic acid sequence in single-stranded form, (b) contacting the test nucleic acid sequence under hybridizing conditions with a solution containing an oligonucleotide probe which is complementary to a defined portion of a standard nucleic acid sequence, (c) immobilizing to a solid support a nucleic acid fragment at least part of which is complementary to said oligonucleotide probe, (d) contacting the solution from step (b) with said solid support, and (e) determining the amount of binding of oligonucleotide probe present in said solution to its complementary nucleic acid fragment on the second solid support, said amount being inversely related to the amount of binding of the oligonucleotide probe to the test nucleic acid sequence.