Abstract: A method of radioimmunoassay for the determination of the primary serum conjugated cholic acid was reported. The antibodies against BSA conjugated cholic acid occurred in rabbits. Cholyglycylhistamine, a derivative of cholic acid, was synthesized and characterized. This derivative was iodinated using Na ¹²⁵I by chloramine-T labelling technique and purified by thin layer chromatography and used as tracers in the RIA. The processes of this method for RIA of cholic acid may be summarized as follows: （1） precipitating the serum with 9 vols. of absolute ethanol; （2） evaporating duplicate ethanol sample of 50—100μl to dryness; （3） redissolving the residue in 0.01M PBS and incubating the samples in the presence of antiserum and ¹²⁵I-iodohistamine cholic acid at 37℃ for one hour; （4） separating bound radioactivity with PEG from the unbound and counting the bound part with a gamma counter. Normal sera were found to contain 2.18±1.3 nmoles/ml. Accuracy was determined by addition of cholic acid to pooled sera with a recovery of 91.5—116.0% and a mean value of 98.1%. Precision was determined by replicated analysis of normal samples, with variation coefficients of interassay and intraassay as 3.3% and 7.2% respectively. Sensitivity has been maintained with the least detectable amount of 1.25 pmoles/tube.