Abstract: The invention relates to a chimeric antibody conjugate comprising an antigen binding region of a non-human antibody and an immunoglobulin constant region which comprises at least one CH domain or epitope thereof, with the proviso that the constant region is not a naturally occurring FC fragment. A bifunctional molecule for use in labelling an antibody derived from a first species, the bifunctional molecule comprising a binding region which binds to the antibody of the first species or to one or more groups provided thereon, and a constant region derived from an antibody of a second species, the constant region comprising at least one CH domain or an epitope thereof. The present invention relates to bifunctional molecules and complexes which are useful as positive control reagents in antibody based diagnostic tests. The present invention also relates to polynucleotides encoding these bifunctional molecules, and to diagnostic assays involving the use of these molecules.