In the last decades the knowledge of the effects of UV radiation on human health, especially in skin cancerogenesis, but also in immunsuppression, photoaging, eye damages, has enlarged strongly. The increasing solar UV radiation and changes in life style strengthen the necessity to identify and quantitate intrinsic biomarkers which are indicative for the individual UV susceptibility and the accumulated individual UV burden. For the risk assessment of potentially deleterious UV effects extrinsic biomarkers have to be developed and tested as personal biological UV dosimeters. One example for such a well characterized biological UV dosimeter is the DLRbiofilm which consists of spores of the bacterium Bacillus subtilis as UV sensitive target. J Epidemiol, 1999; 9 : S78-S83.