(c) UVC irradiation (Kurth et al., 1999) : A second rather easy and safe method is UVC irradiation for inactivating adventitious agents. As distinct from γ irradition, the UVC irradiation has to be performed by using a continuous flow through irradiator. An irradiation time of 8±1 s and a fluence of 0.1 J cm −2 are used normally. The principle of this type of irradiation is a DNA excitation leading to electron transfer (→ 8-hydroxoy-guanine), photohydration (→ cytosin hydrates), photoaddition and dimerization (→ Pyr -Pyr, Thy -Ade, Pyrimidine (6-4) Pyrimidone). UVC irradiation is very effective for selected virus groups, especially for those with single-stranded nucleic acids. The data shown in Table XIII indicate that all tested single-stranded viruses were rather efficiently inactivated with a log clearance ranging from >5.5 to 8. Only the reovirus 3 which has a double stranded RNA shows a lower log reduction (4). Long term growth assays did not reveal any reduction in the ability of the UVC irradiated sera (used at 1%) to support cell growth.