In this study, we examined the effect of 1,3,6,8-tetrachlorodibenzo-p-dioxin (1,3,6,8-TCDD), octachlorodibenzo-p-dioxin (OCDD) and 2,4,6,8-tetrachlorodibenzofuran (2,4,6,8-TCDF) on aryl hydrocarbon hydroxylase (AHH) activity in cultured human lymphoblastoid cells. These compounds are considered to be no or less toxic isomers of polychlorinated dibenzo-p-dioxins (Dioxins) and polychlorinated dibenzofurans (Dibenzofurans). At a concentration of 90 ppb, 1,3,6,8-TCDD induced the enzyme activity about 2 times over the basal (untreated) AHH activity and 2,4,6,8-TCDF reduced the basal enzyme activity by about 40%. At a concentration of 5 ppb, OCDD enhanced the basal AHH activity by about 30%. In case of simultaneous treatment of 1,3,6,8-TCDD, OCDD or 2,4,6,8-TCDF with highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), at a concentration of 7.5 ppb of each compound, the AHH activities induced by 1,3,6,8-TCDD plus 2,3,7,8-TCDD and OCDD plus 2,3,7,8-TCDD were about 25% and 43%, respectively, higher than that induced by 2,3,7,8-TCDD alone. On the contrary, at the same concentration, the enzyme activity treated with 2,4,6,8-TCDF plus 2,3,7,8-TCDD were about 30% less than that treated with 2,3,7,8-TCDD alone. It has been reported that the AHH inducibility of Dioxins and Dibenzofurans correlates well with their toxic potency. Hence, taking the results of this study into account, we should clarify the biological and/or health consequences of the mixed contamination of no or less toxic and highly toxic congeners of Dioxins and Dibenzofurans by animal experiments and epidemiological studies.