Tandem mass spectrometry (MS/MS) allows for the simultaneous quantitation of endogenous and administered stable isotope tracers of vitamin A, provitamin A carotenoids, and their metabolites in human plasma. This information can be used to determine interindividual variations in provitamin A carotenoid bioavailability and bioconversion (bioefficacy), and for estimations of total body vitamin A stores. Since this method only requires one blood sample, its use will facilitate the accurate determination of relative bioefficacy and total body vitamin A stores in human populations and especially in children, in whom repeated blood samples cannot be taken.