Hexabromobiphenyl has been identified by gas chromatography/mass spectrometry (GC/MS) in pooled extracts of adipose tissue samples collected from the general population of the conterminous United States. Mass spectra derived from tissue extracts subjected to gel permeation chromatography were compared with those obtained from an authentic PBB mixture containing 2,4,5,2′,4′,5′-hexabromobiphenyl as the principal component. GC retention times and unique characteristics of the mass spectrum permitted confirmation of identity of the hexabromo isomer. Levels in one tissue sample were determined to be in the 1 to 2 ppb range.