Polychlorinated biphenyls (PCBs) are widespread persistent organic pollutants. The metabolism of PCBs by various organisms involves many steps that can lead to the formation of a wide range of metabolites. These metabolites frequently exhibit a toxicity and biodegradability different than the parent compounds. There is currently little information available about the biological effects of PCB hydroxylated metabolites that can be generated by various organisms and potentially released into the environment. The objective of the present research is to compare the toxicity of selected PCB congeners and their corresponding mono-hydroxylated metabolites. To achieve this objective, the following specific aims were performed: (1) to determine the effect of selected PCBs and PCB hydroxylated metabolites on the growth rate of a model PCB-degrading bacterium, Burkholderia xenovorans LB 400, (2) to determine the microbial toxicity of PCBs and PCB metabolites using the bioluminescent assay Microtox®, and (3) to determine the estrogenicity of PCBs and PCB metabolites using the Yeast Estrogen Screen assay (YES). The effects of a range of PCBs (PCB-2, -3, -8, -9, -30, -35, -36, -39, -61, -68, and -79) and their mono-hydroxylated metabolites on the growth rate of the PCB degrader, Burkholderia xenovorans LB400, were recorded. The results showed that the parent PCBs (50 mg L-1) did not affect the growth rate of LB400 although their hydroxylated metabolites strongly inhibited microbial growth. Using Microtox® assay, Parent PCBs (50 mg L-1) did not exhibit observable toxicity, while their hydroxylated metabolites showed a high level of toxicity (EC50 ranges from 2 mg L-1 to 46 mg L-1). Results using the YES assay also showed that the estrogenicity of hydroxylated metabolites of PCBs (50 mg L-1) was higher than the parent PCBs. The results obtained from the present study show that mono-hydroxylated metabolites of PCBs are more toxic than the corresponding parent PCBs. Because hydroxylated PCB derivatives are produced by a range of organisms and potentially released into the environment, this work raises new concerns associated with the environmental fate of PCBs. / Civil Engineering