Adenoviruses (Ad) are double-stranded (ds) DNA, non-enveloped viruses. There are seven species (A-G) of human Ads with 52 knownserotypes to date. Human Ads cause a broad range of pathologies, ranging from upper respiratory tract infections to persistent urinary tract infections. The main determinant for Ads tropism in vitro is the protruding, antenna-like, fiber protein. The fiberknob is responsible for the main interaction with the attachment receptor of the host cell. Most Ad species use the coxsackie- adenovirus receptor (CAR) as their main attachment receptor. Most species B Ads, however use CD46. CD46 is a cell surface complement regulatory protein, which is expressed on all nucleated cells in humans. Species B Ads exhibit a low seroprevalenc in the human population, making these Ads promising vector candidates for gene therapy. We have studied human Ad species B members, serotypes 7 and 11 (Ad7 and Ad11), as well as their interaction with CD46. Our first experiments showed that all species B Ads use CD46 as their main attachment receptor, with the exception of Ad3 and Ad7. Second, we performed mutational studies of recombinant Ad11p fiberknobs. These studies showed that arginine 279 in the Ad 11 fiberknob is necessary for CD46 binding. Finally we studied the effect of Ad11 binding to CD46. The results indicate that CD46 is rapidly downregulated on the cell surface after Ad11 binding. These results may provide a further understanding of the basic biology and pathology of species B Ads and may also be useful in construction of gene therapy vectors based on species B Ads.