Summary Mice can be immunized with the isolated capsid components of adenovirus (fibre and hexon) and the antibody response is largely thymus dependent. Immunized mice are protected against challenge with a lethal dose of adenovirus and this protection is type-specific for fibre, and mostly type-specific for hexon. The protection is antibody-mediated and there is an absolute requirement for a detectable level of circulating antibody to be present at the time of challenge. Although cell-mediated immunity is insufficient to protect against this particular disease syndrome, the possibility of its importance in protecting against natural adenovirus infection remains.