An analytical method is descrlbed for measuring 2,3,7,8-tetrachiorodlbenzo-p dioxin (TCDD) and other TCDD IsomersIn human tlssue samples using a highly speclfic cleanup procedure and hlgh-resolution gas chromatography/high-resoiution mass spectrometry. The 2,3,7,8-TCDD Is quantlfied by the Isotope dilutlon technique with ["C1,]2,3,7,8-TCDD as the internal standard. Other TCDD isomers are estimated by use of relative response factors. The 1 part-per-trllllon (pptr) ilmk of detectlon is adequate for determlnlng 2,3,7,8-TCDD concentrations present in human adlpose tlssue specimens. Analytical accuracy is demonstrated by the results obtained In analyzing several spiked samples. The method Is shown to be unaffected by a number of potentially Interfering compounds. A series of quailty control material Is used to verify system performance. For samples containing 1.6 pptr 2,3,7,8-TCDD, a coefficient of variation (CV) of 19% is observed. For samples at the 8.4 pptr level, a CV of 11 % is observed.In recent reviews (1-3), the results of quantification of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in biological samples by using various approaches to gas chromatography/mass spectrometry (GC/MS) instrumentation have been summarized. Fish are the most studied group of animals, followed by cattle.