An aminoimidazolyl derivative of rabbit anti-human group·erythrocyte IgG has been synthesised by treatment of the partially purified antiserum with ethyl N-(carbamoylcyanomethyl)acetimidate. The derivative was diazotised and then coupled with a wide range of different protein antigens to give a series of ‘antibody-coupled antigen reagents’ for the red cell-linked antigen-antiglobulin reaction. This new chemical reaction sequence has important advantages over those used hitherto. Examples are given of the preparation and use for the measurement of antibodies in human serum of antibody-coupled antigen reagents derived from α-lactalbumin, β-lactoglobulin and casein.