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A mass outbreak of poisoning occurred in central Taiwan in 1979 due to the ingestion of rice-bran oil contaminated with polychlorinated biphenyls (PCBs), dibenzofurans (PCDFs) and quaterphenyls (PCQs). The incident was called PCB poisoning or Yu-Cheng in Taiwan. The major PCB and PCDF congeners in the toxic oil and in the blood and tissues of the poisoned patients were characterized by gas chromatography and gas chromatography-mass spectrometry using highly efficient glass capillary columns. The levels of toxic agents in the rice oil samples collected from the factory and school cafeterias and the families of the poisoned patients are in the range of 53 to 99 ppm, 0.18 to 0.40 ppm and 25 to 53 ppm for PCBs, PCDFs, and PCQs, respectively. The blood samples of 165 patients collected 9 to 18 months after the onset of poisoning contained 10 to 720 ppb of PCBs, with a mean value of 38 ppb. The blood samples of 10 patients collected 9 to 27 months after poisoning contained 0.02 to 0.20 ppb of PCDFs. Comparative rates of elimination of some PCB congeners from the blood of patients were studied. Various tissues from a patient who died 2 years after poisoning were analyzed for PCBs, PCDFs and PCQs. The intestinal fat contains the highest level of PCBs, while the liver contains the highest concentration of PCDFs. The PCB congeners retained in the tissues either do not have adjacent unsubstituted carbon atoms or have a pair at ortho-meta positions of the biphenyl ring. The major PCDF congeners retained in the tissues were 1,2,3,4,7,8-hexachloro-DF, 2,3,4,7,8-pentachloro-DF and 1,2,4,7,8-pentachloro-DF. The former two congeners, especially 2,3,4,7,8-pentachloro-DF, are very toxic PCDFs; they may play important roles in the etiology of Yu-Cheng.