A new enzymatic fluorimetric assay for the determination of total serum bile acids is described. The enzyme 3 alpha-idroxysteroid NAD oxireductase by oxydising the free OH in position 3 alpha of bile acid molecule, reduce NAD previously added. Afterwards the hydrogen of the generated NADH is transferred by a diaphorase to resazurin to yield the fluorophore resorufin proportionally to the amount of bile acids in the sample. The assay is simple, accurate, without bilirubin's influence and useful for a routine use because the analysis time is not different from the more common laboratory tests.