The invention relates to a method for determining antigen-specific antibodies of immunoglobulin class M in the presence of immunoglobulins of class G and/or rheumatoid factor in body fluids by incubation with at least two different receptors R1 and R2 and optionally additional receptors, wherein a bonding partner in polymer form is a substantial constituent of R2 and the disturbing effects of IgM antibodies with the same antigen specificity are suppressed by bonding partners in monomer form. The invention also relates to a reagent for determining an antigen-specific antibody of immunoglobulin class M and the use of bonding partners in monomer form in suppressing the disturbing effects of IgG antibodies upon determination of antigen-specific IgM antibodies.
