Currently available live spore vaccines against anthrax in animals have many drawbacks, one of which is their presumed inability to induce a long lasting immunity. In the present study we compared the immunological memory after a protein vaccination with DNA vaccinations in sheep. The antigen used was the protective antigen (PA83) of Bacillus anthracis. Sheep were vaccinated three times with either PA83 plus alhydrogel, or with one of four different plasmid DNA formulations, which all encoded either the full-length PA83 or its domain 4. Two pDNA formulations included Vaxfectin adjuvant, the other two were injected in PBS without adjuvant. Initially, the antibody titres of protein vaccinated sheep were significantly higher than the titres of pDNA vaccinated sheep. After 5 months, however, the antibody titres of protein vaccinated sheep had dropped remarkably, while the titres of all four pDNA vaccinated groups were either stable or had increased. Humoral responses of sheep immunised with pDNA formulated with Vaxfectin adjuvant were higher than the responses of the corresponding groups that received pDNA in PBS only.