ON studying the distribution of 14C from nuclear weapon tests in a wide range of human tissues, it was observed that significant modification of 14C/12C isotopic ratio had occurred, largely as a function of variable carbon renewal rates1,2. As a parallel exercise, 13C/12C isotope ratios have been assayed, 13C and 12C being the nonradioactive isotopes of carbon which are assumed to be unaffected by nuclear testing. In principle, however, it is feasible that the differing biochemistries and carbon renewal rates of various body tissues could induce measurable stable isotope ratio changes (fractionation) as a result of the discrepancy in nuclear mass between 12C and 13C. This inequality leads to a difference in basic thermodynamic properties manifested as zero point energy, diffusion coefficients and kinetics of reaction for 12C- and 13C-substituted molecules. Thus, in reactions at equilibrium or those kinetically controlled, preferential selection of either the light or heavy isotope, and hence 13C/12C ratio change, may occur. We report here the results of over 80 measurements on human tissue samples which have increased the data available on natural baseline isotope ratios. This is particularly relevant because of the increasing use of stable, rather than radioactive, tracers in metabolic studies.