Lambs were infected orally at 14 weeks of age with either Taenia hydatigena, T ovis or Echinococcus granulosus eggs. To induce high levels of resistance to oral reinfection, they were then injected intramuscularly with eggs of the homologous species four weeks and eight weeks later. The degree of immunity was then tested by oral challenge with eggs from either the homologous or an heterologous species. Autopsy results indicated that prior infection with an homologous species induced a very high degree of immunity to challenge infection. Prior infection with all combinations of heterologous species resulted in a lesser degree of immunity, which appeared to have acted on the establishment phase of development and did not influence cyst survival.