Abstract: In this paper iodine labelling of cholylglycylhistamine (CGH) is discussed. Theimmunoactivity of monoiodo-CGH (B_0/T=70%) is greater than that of diiodo-CGH(B_0/T=45%). Antiserum quality of cholylglycine(CG) (affinity constant K crossreactivity and titer) is studied. The relation between the quality and radioimmun-oassay is shown by logit-log plot. Antiserum binding is effected by protectionproteins. The CG standard is prepared with special serum.