Abstract: Antibodies (Ak), in particular autoantibodies, such as autoantibodies against thyroid peroxidase (TPO), are determined by a procedure in which the sample to be investigated is reacted with two further, in particular monoclonal antibodies and an antigen. The two further antibodies are an immobilised antibody (Ak.sub.imm) and a freely labelled antibody (Ak*) against the antigen (Ag), and the latter is preferably used in the form of a crude, natural antigen, for example in the form of an organ extract. The presence of the antibodies (Ak) to be determined is evident as a disturbance of the formation of a sandwich of the two added antibodies (Ak.sub.imm and Ak*) with the added antigen (Ag) and as a consequent reduction in the binding of the label to the solid phase.