Herein is reported a method for the determination of the amount of a bivalent antibody in a serum or plasma sample obtained from a non-human experimental animal, whereby the antibody comprises one or more mutations in the Fc-region compared to the corresponding wild-type Fc-region that has a sequence of SEQ ID NO: 01, 02, or 03, wherein the method comprises the following steps a) immobilizing a non-antibody polypeptide to which more than one copy of the antigen of the antibody is covalently conjugated on a solid surface, b) incubating the immobilized antigen with the sample to form an immobilized antigen-antibody complex, c) incubating the immobilized antigen-antibody complex with the antigen conjugated to a detectable label to form an immobilized ternary complex, and d) determining the amount of the antibody by determining the amount of the detectable label in the immobilized ternary complex.