Abstract: A method of qualitatively or quantitatively indicating immunoglobulin whose Fab-portion is bound to or is caused to bind to an antigen or a hapten or indicating the antigen or hapten which is bound to or caused to bind to the Fab-portion of the immunoglobulin, the immunoglobulin being labelled -- before or after binding to the antigen or hapten -- with one or more analytically indicatable atoms or groups by contact with a polypeptide labelled with the analytically indicatable atom or group or atoms or groups, wherein the immunoglobulin belongs to the IgG-class and the polypeptide is one which is obtained from microorganisms and which is able to bind itself to the Fc-portion of said immunoglobulin belonging to the IgG-class. The method utilizes an auxiliary agent for labelling immunoglobulin and a reagent for quantitatively or qualitatively indicating antigens or haptens.