The invention relates to a method for the quantitative or semi-quantitative analysis of the free binding capacity of a protein, especially of albumin. The aim of the invention is to provide an indirect method in an aqueous test solution that can be carried out as simply and quickly as possible in serial tests. The inventive method comprises the following steps: a) providing a solution (L1) that contains an at least approximately defined amount of the test protein (A) and a solution (L2) that contains the same at least approximately defined amount of a reference protein (D); b) adding to the two solutions (L1 and L2) the same defined amount of a marker substance (M) each, said marker substance being capable of binding to a defined binding site of the test protein (A) and of the reference protein (D), and adding said marker substance to the solutions in such an amount that only substantially all binding sites for said marker substance are saturated; c) allowing the marker substance (M) to bind to the free binding sites of the respective protein (A or D), thereby forming a protein/marker substance complex (A:M, D:M), by incubating the mixture for a sufficient time; d) separating the marker substance (M) not bound from the respective protein/marker substance complex (A:M; D:M); e) determining the respective amount (MAung; MDung) of the marker substance not bound or a physicochemical modification (SAung; SDung) that is proportional thereto; and f) determining the binding capacity (K) of the test protein according to the formula (a).
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