An analytical procedure for the isomer-specific quantification of polychlorinated dibenzo-p-dioxins and dibenzofurans in samples of human and animal origin is described. It consists of a large-scale clean-up including various kinds of column chromatography and high-resolution gas chromatographic/mass spectrometric quantification referring to internal 13C-labelled standards. By additional application of gel-permeation chromatography and the use of a retention gap, detection limits in the low ppq range (10(-15) were achieved.