Vaccination has relied, in general, on two technologies for the production of antigenic material suitable for the generation of a protective immune response; live infectious agents and inactivated or subunit vaccine preparations. Live infectious agents generally are most efficacious, but there is always a risk factor when using this type of vaccine. The major advantage of inactivated and subunit preparations is their safety; however, limited efficacy and duration of immunity and/or high cost may limit the usefulness of these types of vaccines. DNA vaccines represent a new and potentially powerful approach to the development of subunit vaccines.