The aim of this study was to provide information for improving risk assessment of viral contaminants in bivalves. The persistence of viable adenovirus type 35 (Ad35) after controlled contaminations of blue mussels, Mytilus edulis, and oysters, Ostrea edulis, was studied. Bivalves, kept in running seawater at two different temperatures (4 and 18 degrees C) were sampled after 1, 3, 7, 14, 21, 35, 42, 49, 56, and 70 days. Virus particles were separated from the gills and the digestive gland through ultra high-speed centrifugation. Qualitative PCR analyses of DNA in the virus extracts showed that Ad35 was detectable for 6-10 weeks and quantitative real-time PCR verified a gradual but not linear decrease in copy numbers, within this time interval. The virus genome was detectable to the same degree on the gills as in the digestive gland. When viral extractions were inoculated on A549 cells to investigate the cytopathic effect (CPE) it was shown that Ad35 stayed infectious in oysters, kept at 4 degrees C, for about six weeks, which was double the time compared to that for mussels. The detection of the viral genome exceeded the persistence of their infectivity, in most cases with 4-6 weeks. The data were highly variable and the sporadic occurrence of high numbers of accumulated viruses and their remaining infectivity is seemingly a significant factor regarding food safety.