A new method for the selective determination of the main serum bile acids has been developed. Serum samples with added 14C-labeled bile acid were submitted to deproteinization, alkaline hydrolysis, methylation, and were then chromatographed on alumina before acetylation with 2 microliters of [3H]acetic anhydride. Excess reagent was eliminated by evaporation; elimination of residual tritiated contaminants and separation of the doubly labeled bile acid derivatives were obtained by thin-layer chromatography, column chromatography on Lipidex 5000, and crystallization. The sensitivity of the method is about 10 pmol of each bile acid. Analyses of seven sera with normal or elevated concentration of bile acids by the proposed method and gas-liquid chromatography showed a close correlation (r = 0.94; slope = 0.93).