A method of quantifying multiple antigen-specific immunoglobulins in a test sample, the method comprising the steps of; (i) assaying binding of a series of samples containing immunoglobulin of known antigen reactivity to multiple antigen components or fragments thereof immobilised on a first solid support, (ii) comparing the level of binding in step (i) with the known reactivity to produce a dose response curve for each antigen component or fragment thereof, (iii) assaying binding of a serial dilution of a reference immunoglobulin sample of the same immunoglobulin subtype as that used in part (i) with a known total amount of immunoglobulin to a serial dilution of anti-immunoglobulin antibodies, fragments or derivatives thereof, immobilised on the first, or a second solid support, (iv) comparing the level of binding in step (iii) with the known total amount of reference immunoglobulin to produce a binding capacity curve for each anti-immunoglobulin antibody, fragment or derivative dilution, (v) comparing the dose response curves produced in step (ii) with the binding capacity curves produced in step (iv), identifying the binding capacity curve that most closely matches the dose-response curve for each antigen or fragment thereof, and assigning the identified binding capacity curve as a binding capacity curve to each antigen or fragment thereof, (vi) assaying binding of antigen-specific immunoglobulin in the test sample to the antigen components or fragments thereof immobilised on the first, second, or a third solid support, and (vii) comparing the level of binding in step (vi), with respect to each individual antigen or fragment thereof, to the binding capacity curve assigned to that antigen or fragment thereof in step (v) and quantifying the level of antigen-specific immunoglobulin present in the test sample. (i) assaying binding of a series of samples containing immunoglobulin of known antigen reactivity to multiple antigen components or fragments thereof immobilised on a first solid support, (ii) comparing the level of binding in step (i) with the known reactivity to produce a dose response curve for each antigen component or fragment thereof, (iii) assaying binding of a serial dilution of a reference immunoglobulin sample of the same immunoglobulin subtype as that used in part (i) with a known total amount of immunoglobulin to a serial dilution of anti-immunoglobulin antibodies, fragments or derivatives thereof, immobilised on the first, or a second solid support, (iv) comparing the level of binding in step (iii) with the known total amount of reference immunoglobulin to produce a binding capacity curve for each anti-immunoglobulin antibody, fragment or derivative dilution, (v) comparing the dose response curves produced in step (ii) with the binding capacity curves produced in step (iv), identifying the binding capacity curve that most closely matches the dose-response curve for each antigen or fragment thereof, and assigning the identified binding capacity curve as a binding capacity curve to each antigen or fragment thereof, (vi) assaying binding of antigen-specific immunoglobulin in the test sample to the antigen components or fragments thereof immobilised on the first, second, or a third solid support, and (vii) comparing the level of binding in step (vi), with respect to each individual antigen or fragment thereof, to the binding capacity curve assigned to that antigen or fragment thereof in step (v) and quantifying the level of antigen-specific immunoglobulin present in the test sample.