To present short protein sequences to the host immune system a foreign epitope has been expressed on the surface of the adenovirus virion as part of the hexon. As the trimeric hexon constitutes 240 out of the 252 capsomers of the virus, the foreign epitope is repeated 720 times on the virion surface. An eight amino acid sequence from the major antigenic site in the VP1 capsid protein of poliovirus type 3 was engineered into two regions of the adenovirus type 2 hexon. The two loop regions chosen to accommodate the foreign sequences are exposed on the surface of the virion, show sequence variation between serotypes and are the sites of interaction with neutralizing antibodies. Virus with substitutions in loop I had wild-type growth characteristics, whereas virus with substitutions in loop II grew poorly. Adenoviruses with poliovirus sequences in loop I were recognized and efficiently neutralized by antisera specific for the poliovirus sequence; an antiserum raised against the adenovirus with the poliovirus insert specifically recognized the VP1 capsid protein of poliovirus type 3. It is therefore feasible to alter the surface properties of the adenovirus virion and in doing so to manipulate the immune response to this virus.