In order to identify suitable adenoviral vectors for efficient delivery of transgenic proteins and peptides to the intestine, the ability of adenovirus types 5 and 41 (an enterotropic serotype) to bind to and enter undifferentiated and differentiated enterocytes was assessed. FACS analysis showed no significant difference between the virions in their ability to bind to undifferentiated Caco-2 cells as 81.6% of the cellular population bound adenovirus 5 (Ad 5) and 79.8% bound Ad 41. Both virions were also efficiently internalized in this cell type as 99.6% of the cells took up Ad 5, while 95.9% took up Ad 41. In studies with differentiated enterocytes, probable targets for oral gene delivery but rather resistant to adenovirus-mediated gene transfer, 28.4% of the population internalized the Ad 5 vector and less than 10% bound the virus. Adenovirus 41 was efficiently internalized in differentiated enterocytes as 89.6% of the cellular population took up the virus while 37.4% bound the virus. These results were consistent with those observed in vivo in rat jejunum. Thus, molecularly engineered Ad 41-based recombinants could be highly efficient vectors for delivery of transgenic proteins to differentiated enterocytes.