Six types of octadecachloroquaterphenyls (ODCQPs) were prepared by chlorination of the corresponding quaterphenyls (QPs), and the QP skeletons of polychlorinated quaterphenyls (PCQs) in the blood of "Yusho"patients were investigated. The results are as follows : (1) The six types of ODCQPs showed characteristic mass spectra. (2) PCQs in the blood of Yusho patients were found to be a mixture of six types of PCQ isomers by gas chromatography and mass spectrometry. (3) The ratios of six types of PCQ isomers in the Kanechlor 400 used as the thermotransfer medium and in the blood of Yusho patients were different.