Vaccination of intermediate hosts will effectively reduce the prevalence of human echinococcosis. The rate of progress can be enhanced by concurrent tactical treatment of definitive hosts. The oncosphere of Echinococcus granulosus contains a specific protein which when expressed on the oncosphere surface will combine with specific antibody In the presence of complement, damage is caused to the plasma membrane, resulting in failure of the oncosphere to develop into a metacestode. Immunisation of sheep with oncosphere secretions or with an oncosphere homogenate results in 99% resistance to a challenge infection of the sheep with E. granulosus eggs. Immunisation of sheep with the specific purified protein results in 92% resistance. The gene coding for the protein has been isolated from an E. granulosus oncosphere cDNA library and expressed in E.coli as a fusion protein with glutathione-S-transferase. A monoclonal antibody to an epitope on the E. granulosus portion of the molecule is able to bind to the surface of oncospheres and to prevent their development to metacestodes in vitro. Immunisation of sheep with the fusion protein has given 97 and 98% resistance to a challenge infection in two separate trials. The shelf-life of formulated vaccine is at least 12 months, and the immunity generated by two injections given 1 month apart persists for at least 12 months. A homologue of the gene has been identified in E. multilocularis. The vaccine has the potential to be used as a tool for control of transmission of E. granulosus through its natural intermediate hosts (particularly domestic ungulates) as part of hydatid control programmes. Traditional methods of hydatid control (anthelmintic treatment of dogs) require regular treatment within the prepatent period for prolonged periods (20-30 years) to achieve satisfactory control, and often sociological factors to do with dog-feeding, dog ownership and dog-control prevent such an achievement. By controlling the disease in both the definitive host and the intermediate host, the shortcomings of either procedure can be overcome, resulting in a cost-effective desired result within a practical time period.