1. A method has been developed for the separate and specific estimation of cholic and chenodeoxycholic acids in human blood sera. 2. The method differs from previously reported procedures with respect to the methods used for the extraction, hydrolysis, fractionation and final, colorirnetric estimation of the bile acids. 3. A new modification of the Pettenkoffer reaction has been developed which seems to be specific for bile acids possessing either a hydroxyl group or a double bond at position-7. 4. Previously reported values for the concentrations of cholic acid or chenodeoxycholic acid in normal and hypercholesterolaemic human blood sera are erroneous by virtue of being too high. Estimates are given for the maximum concentrations of these bile acids which might be expected to occur in these sera. 5. Values are reported for the concentrations of cholic and chenodeoxycholic acids in sera from humans with obstructive jaundice, cirrhosis and hepatitis and the values compared with corresponding values obtained by other authors in recent years. The clinical significance of these measurements is discussed. 6. The logarithms of the concentrations of cholic and chenodeoxycholic acids in pathological sera have been shown to be normally distributed. 7. The state of conjugation of cholic and chenodeoxycholic acids in certain pathological sera seems to correlate with their state of conjugation in bile from the same types of subject. 8. It has been shown that in some cases the extraction of serum with ethanol does not result in all the cholic acid in the serum passing into the ethanolic extract. The significance of this is discussed. 9. The feeding of cholic acid to rats under conditions which result in hypercholesterolaemia also results in the appearance of cholic acid in the serum of the rats. 10. The occurrence and possible structure of α-lagodeoxycholic acid in rabbit bile is discussed.