AbstractIncorporation of amino acids labeled with stable isotopes in apoliproproteins is used to estimate kinetic aspects of lipoprotein metabolism. In this study two deuterated tracers, [5,5,5‐2H3]leucine and [ring‐2H5]phenylalanine, were compared. Isolation and acid hydrolysis of apolipoproteins are required for mass spectrometric analysis. When apolipoprotein B100of very low density liproproteins was prepared with this procedure, a loss of deuterium was observed on deuterated phenylalanine with 10 and 6MHCl hydrolysis but not with deuterated leucine or when 4 M HCl hydrolysis was used. This study stre sses the effect of acid hydrolysis on [ring‐2H5]phenylalanine. This tracer must be used with caution in studies of specific protein synt