An indirect fluorescent antibody test (IFAT), based on schizont-infected lymphoblastoid cells, was applied to study the course of antibody production in adult sheep inoculated with attenuated, in vitro grown, Theileria lestoquardi (Theileria hirci) infected cells. Bright fluorescence of the intracellular schizonts could first be demonstrated 15 days after inoculation. A 32-64-fold rise in antibody titres was recorded 1 month after infection, and substantial titres were still observed 90 days after inoculation. Fluorescence was absent with negative control sera and background staining was minimal. No serological cross-reactions were detected with sheep sera positive for Babesia motasi, Babesia ovis or Toxoplasma gondii. Results obtained did not differ when antigens prepared from three different strains of T. lestoquardi infected lymphoid cells were compared. Testing for reactivity to non-pathogenic Theileria species of sheep revealed a low degree of cross-reaction of a Theileria ovis and a Theileria separata antiserum to T. lestoquardi antigen. Cross-reactions were also observed with bovine sera positive for Theileria annulata and Theileria parva. Moreover, T. lestoquardi positive sera reacted almost equally strongly with bovine T. annulata antigen as with their homologous antigen, whereas cross-reaction with bovine T. parva antigen was less pronounced. These results indicate a close antigenic relationship between ovine T. lestoquardi and T. annulata of cattle.