In man there are four main bile acid fractions and within each fraction there is the possibility of at least three bile acid moities - two conjugated and one unconjugated. Bile acids thus present a considerable challenge to radioimmunoassay techniques. Few of the antisera described to date are satisfactory in that they do not show equal reactivity to each of the moities to be assayed, and many have unacceptable cross reactivity properties. Clearly there is need for caution and development in this field. The first application of radioimmunoassay (RIA) techniques to the measurement of serum bile acids was made by Simmonds, Korman, Go and Hofmann in 19731. Because of its unique sensitivity and ease of application, RIA has been used not only to determine the increased serum bile acid levels found in liver disease, but also to monitor the clearance of bile acids from the peripheral circulation of normal subjects, to provide 24 hour serum bile acid profiles in normal subjects and even to assay the low serum bile acid levels found in patients in whom the bile acid enterohepatic circulation has been interrupted. Some 20 preparations of bile acid antisera have been described to date and commercial kits are now available, for RIA of each of the 4 major bile acid fractions found in human sera. Considerable differences however, in the specificity of these antisera are indicated by their reported cross reactivities and the analytical validity of their use is often questionable.