Abstract: Radioimmunoassay (RIA) by means of block agent is designed to measurethe concentration of Cholylglycine (CG) in human serum. This is a direct assaymethod which is simple and can give result very quickly. The CG RIA Kit consists of the following reagents: (1) ~(125)I--cholylglychistamineas labelling antigen, radiochemical purity>90%. (2) The CG standards preparedwith normal man serum from which hormone has been removed. The concentra-tions are as follows: 0, 25, 100, 250, 1000 and 4000 μg/dl. (3) CG antiserum(rabbit ~#16), with an affinity constant of 2.6×10~8 liters/mole. (4) The blockagent which is 0.5% ANS (8--anilino--1--naphthalene sulphonic acid) in phosphatebuffered saline. (5) The PEG (polyethylene glycol) which is used to separatebound labeled CG from the free CG. The sensitivity of the CG RIA is found to be 10 μg/dl. The precision asevaluated by assaying of normal samples is 4.7% for intraassay and 12.5% forinterassay. The accuracy was determined by addition of CG to serum pools witha mean recovery of 109%. The Kit is stable and reliable,