Abstract: The invention provides a method of measuring the affinity of first and second biomolecules in which a first biomolecule is tethered by a first tether portion having a first tether portion length and a second biomolecule is tethered by a second tether portion having a second tether portion length, the method comprising determining binding of adjacent first and second biomolecules to each other, varying at least one of the first and second tether lengths and determining binding of the first and second biomolecules. The invention also provides apparatus suitable for use in the method of the invention.