Sensitive tritiated radioimmunoassay was developed for conjugated chenodeoxycholic acid, using immunogen prepared by the mixed anhydride method. The obtained molar bile salt-BSA ratio in the immunogen was 19:1. The distinguishing features of the immunization procedure were a preliminary vaccination of the animal with antitubercular vaccine (VDS), and the administration of very small doses of immunogen (50 micrograms). Assay sensitivity for this bile salt fell in the picomole range with the standard curve extending from 1.5 to 150 pmol. Specificity of the antiserum was compared with that of the commercially available "Glycochenodeoxycholic acid RIA kit" (Nordiclab Oy, Oulu, Finland), and proved to be satisfactory. Fasting serum conjugated chenodeoxycholic acid concentration in 25 healthy subjects and 15 patients with cirrhosis was 0.63 mumol/l and 43.05 mumol/l, respectively. The assay was performed on unextracted sera.