2,3,4',6-Tetrachlorobiphenyl (tetraCB) and the corresponding 14C-labelled compound (14C-tetraCB) were synthesized. Two reference compounds, 4-methylthio- and 4-methylsulphonyl-2,3,4',6-tetrachlorobiphenyl were also prepared and characterized. TetraCB and 35S-cysteine were given to groups of female mice. Formation of methyl[35S]sulphonyl-tetraCB was indicated by the presence of extractable sulphuric acid-soluble radioactivity in lung, liver, kidney and fat of the tetraCB-treated mice. As demonstrated by gel permeation chromatography followed by gas chromatography-mass spectrometry, the tissues of the tetraCB-treated mice contained mainly methylsulphonyl-tetraCB, minor amounts of tetraCB and traces of methylthiotetraCB. The major compound present in lung was 4-methylsulphonyl-tetraCB, indicating the presence of specific binding sites for this metabolite in lung tissue. According to autoradiography of mice injected with 14C-tetraCB, these binding sites were present mainly in the tracheo-bronchial mucosa.