Polychlorinated biphenyls (PCBs) are industrial chemicals that have been released into the environment, resulting in widespread and persistent contamination. PCBs exist as 209 different congeners depending on the chlorine substitution on the biphenyl rings, and the physical properties and toxic effects of different PCB congeners may be structure-dependent. It has been suggested that environmental contaminants may cause DNA damage directly and/or by generation of reactive species such as electrophilic metabolites or free radicals (Xue and Warshawsky, 2005). Potential of PCBs for genotoxicity has been controversial (Belpaeme et al, 1996). The present study was designed to determine genotoxic effects of ortho-substituted, noncoplanar congener, 2,2′,5,5′-tetrachlorobiphenyl (PCB 52), and a non-ortho-substituted coplanar congener, 3,3′,4,4′-tetrachlorobiphenyl (PCB 77) on cultured human peripheral lymphocytes. DNA damage was assessed by use of the comet assay (alkaline single-cell gel electrophoresis). The six healthy male non-smoking donors (mean age 29.3±2.3 years) were used in the study. Subjects had not been exposed to radiation or drugs six months prior to the study. Whole peripheral blood samples were collected by venipuncture. The isolation of lymphocytes was performed by centrifugation in a density gradient of lymphorep (15 min, 280xg) and the cells washed with RPMI-1640 medium supplemented with 10% fetal calf serum. Cell viability was determined with the use of 0.4% tryphan blue. After cell cultures were prepared, test groups were treated with different concentrations of PCB 52 (0.2 and 1 μM) and PCB 77 (1 and 10 μM), and vehicle control group was treated with DMSO for 1 hour at 37 ⁰C in a humidified carbon dioxide incubator. DMSO concentration in the culture medium was not more than 0.1% (v/v). Positive control group was treated with 50 μM H2O2 for 5 min on ice. Fifty cells per slide and two slides per sample were scored to evaluate DNA damage for each concentration of the tested PCB congeners. Results were statistically analyzed by Student’s t-test. The cells were visually classified into four categories on the basis of extend of migration such as undamaged (UD), low damaged (LD), moderate damaged (MD) and high damaged (HD). Total comet scores (TCS) were calculated as: 1 X HD + 2 X LD + 3 X MD + 4 X HD. The results obtained were compared against to negative control group. The highest concentration of PCB 52 and 77 significantly increased DNA breakage in human lymphocytes (p<0.05). However, lower concentrations of either PCB congeners did not produce any significant DNA damage in the comet assay. Our results indicate that both noncoplanar PCB 52 and coplanar PCB 77 increased DNA damage, but the planarity of the PCB congeners did not affect the direction of action, although the ortho-substituted congeners were found to be more potent.