An immunization and infection experiment using 12 sheep was conducted to study the immune responses elicited by an experimental vaccine consisting of Toxoplasma gondii antigens incorporated into immunostimulating complexes (iscoms). Five sheep were immunized subcutaneously with Toxoplasma iscoms. Two doses were given, with a 6 week interval, and 22 days after the second immunization, these five sheep and five non-immunized sheep were inoculated orally with T. gondii oocysts. The two remaining animals served as non-immunized, uninfected controls. The antibody response was analysed by an indirect fluorescent antibody test detecting IgM and an enzyme-linked immunosorbent assay detecting IgG. The first immunization induced low levels of both IgM and IgG, and the second resulted in high levels of IgG but no marked IgM response. After infection, a further increase in IgG was observed in the immunized animals. In the non-immunized sheep, substantial IgM and IgG levels were detected following infection. Immunoblotting analysis indicated that the antibody response to immunization was directed against the same T. gondii antigen as the early antibody response after infection in the non-immunized sheep. Antibodies recognizing the P30 antigen appeared first, followed by antibodies to P22 and other antigens which were probably also of membrane origin. Lymphocyte stimulation tests were performed 15 and 21 days after the last immunization and 105 days after infection. Significant antigen-induced proliferative responses were observed after immunization as well as after infection.