Metabolism of singly or multiply "C-labeled substrates leads to the production of molecules that contain I3C atoms at various positions. Molecules differing only in the number of isotopic atoms incorporated are referred to as mass isotopomers. The distribution of mass isotopomers of many molecules can be measured by gas chromatographylmass spectrometry after chemical derivatization. Quantification of metabolite mass isotopomer abundance resulting from biological processes necessitates correction of the measured mass isotopomer distribution of the derivatized metabolite for contributions due to naturally occurring isotopes of its elements. This correction must take into account differences in the relative natural abundance distribution of each mass isotopomer (skewing). An IBM-compatible computer program was developed which (i) calculates the natural abundance mass isotopomer distribution of unlabeled and labeled standards given the molecular formula of the derivatized molecule or fragment ion, and (ii) calculates the natural abundance mass isotopomer distribution of the singly and multiply labeled molecule or fragment via non-linear fitting to the measured mass isotopomer distribution of the unlabeled molecule or fragment. The output of this program is used to correct measured mass isotopomer distributions for contributions from natural isotope abundances and to verify measured values for theoretical consistency. Differences between predicted and measured unlabeled and I3C-labeled isotopomer distributions for hydroxamate di-t-butyldimethylsilyl (di-TBDMS) derivatized pyruvate were measured. The program was applied to the mass isotopomer distribution of glucose labeled from [U-'3C3]glycerol and of fatty acids labeled from [ U-'3C,jglucose and either [2-I3C,] acetate or [U-'3C,]acetate. In some of these cases, the measured mass isotopomer distributions corrected by the program were different from those corrected by the classical technique. Implications of these differences including those on the calculation of glucose production due to gluconeogenesis in isolated perfused rat liver are discussed.