AbstractThe rice oil ingested by the patients with Yusho and their blood, liver, and adipose tissue were analyzed for individual congeners of polychlorinated biphenyls (PCBs) and polychlorinated dibenzofurans (PCDFs). The individual congeners identified were examined for accumulation in the liver of monkeys and rats, inducing activities of benzo[a]pyrene 3‐hydroxylase, benzphetamine demethylase, and DT‐diaphorase in rats, and gravimetric changes of the thymus and liver in rats. Among the six PCB congeners detected in Yusho patients, 2,3,4,5,3′,4′‐hexa‐CB seems to be the compound most related to Yusho judging from its strong enzyme‐inducing activities in the liver and the thymus atrophy and liver hypertrophy caused by feeding it to rats. PCDF congeners identified in the patients' tissues showed a stronger toxicity in rats than these PCBs, exhibiting stronger enzyme induction activities and gravimetric changes of the tissues. These PCDF congeners, especially 2,3,4,7,8‐penta‐CDF, were also very accumulative in the liver. Therefore, they are considered as the most important etiologic agents for the current symptoms and signs