A quantitative analytical method has been developed to analyze methyl esters of blood fatty acids derived from human subjects fed deuterium-labeled fats. The GCMS computer method provides for the analysis of the fed deuterium-labeled fatty acids, the naturally occurring blood fatty acids and new fatty acids formed by chain elongation or shortening of the fed labeled fats. Approximately 20 fatty acids including 16, 17, 18 and 20 carbon chain acids were analyzed with a relative standard deviation of 0.02 at the microgram level and a sensitivity of less than one nanogram. The method uses capillary GC to separate the fatty acid esters and isobutane chemical ionization mass spectrometry with multiple ion detection to determine the isotopic constituents of the GC peaks. The technique provides for the determination of overlapping GC peaks labeled with 2, 4 and 6 deuterium atoms and makes extensive use of computers both for data acquisition and processing.