In April 2006, 20 years have passed since the explosion of the fourth block fo the Chernoby nuclear power plant (CNPP). This accident affected millions of people, and large territories were contaminated by radionuclides. As a result, background radiation levels increased, and people from contaminated territories are living constantly in low dose radiation conditions. The aim of this study was to assess the direct and bystander effects from low level y-radiation and from serum samples from CNPP accident victims on human blood lymphocytes and keratinocytes in vitro. The possibility to modify these effects using radioprotective substances was also investigated. Bystander effects induced by laser radiation was also studied. The results have shown that melanin, melatonin and a-tocopherol were able to decrease direct and bystander radiation effects. Melatonin showed the best protective effect, whereas a-tocopheror showed the least proctection. Serum samples from people affected by the Chernobyl accident, even 20 years after the accident, could induce micronuclie formation and decrease the viability of human keratinocytes. A direct correlation was shown between the frequency of aberrations in human peripheral blood lymphocytes from victims of the Chernobyl accident and the level of bystander factors in their blood serum. The most affected group were the Chernobyl liquidators, who were exposed to the highest radiation doses. Radioprotective substances were not able to protect cells from these bystander factors. In general, the present study has aided in our understanding of the nature of bystander effects
