The effect of polychlorinated biphenyls (PCBs) on Vero cell proliferation was investigated, with the attempts to assess the possible hormetic dose-response and to compare their structure-dependent toxicity. Both PCB congeners revealed low doses stimulation in our experiment. However, significant cytotoxicity was only observed in PCB 52 concentrations larger than 0.1 mu g ml(-1), while there was no significant inhibition in PCB 77-treated cells at concentrations selected. Furthermore, the time-dependent cytotoxic trends were different. The comparison between PCB 52 and PCB 77 indicated that the cytotoxic mechanisms involved in coplanar or non-coplanar PCB congener exposure were different, and this difference might be associated with individual genotoxicity and the release of contact inhibition, respectively. (C) 2010 Elsevier B.V. All rights reserved.