To determine the cause of the extremely high accumulative potency of OCDD in human tissues, EROD activity and accumulation of 2,3,7,8-chlorine substituted PCDDs and PCDFs in liver of C57BL/6 and DEAR mice administered these chemicals orally were measured during a period of 28 days. Consequently, in both C57BL/6 mice with high EROD induction and DBA/2 mice with low EROD induction, there was no high accumulation of OCDD similar to that observed in human tissues. In addition, the C57BL/6 mice accumulated larger amounts of these chemicals in their liver than did the DBA/2 mice.