We are developing a technology based on the combined application of riboflavin (RB) and light for inactivating pathogens in blood products while retaining the biological functions of the treated cells and proteins. Virus and bacteria reduction measured by tissue culture infectivity or colony formation with UV light alone and in combination with RB yield equivalent results. The effects of RB as a sensitizing agent on DNA in white cells, bacteria and viruses in combination with UV light exposure have been evaluated. UV-mediated DNA degradation in Jurkat T cells and leukocytes in plasma as measured by the FlowTACS assay was significantly increased in the presence of RB. Agarose gel electrophoretic analysis of DNA in Escherichia coli and leukocytes in plasma demonstrated enhanced DNA degradation in the presence of RB. UV light in combination with RB prevented the reactivation of lambda phage compared with samples irradiated in the absence of RB. UV-mediated oxidative damage in calf thymus DNA was also enhanced in the presence of RB. These observations clearly demonstrate that the presence of RB and UV light selectively enhances damage to the guanine bases in DNA. These data also suggest that the type and extent of damage to DNA for virus in the presence of RB and light make it less likely to be repaired by normal repair pathways available in host cells.