SINCE the reports of Tsur1 and Brocklesby and Hawking2 on the multiplication of Theileria annulata schizonts in spleen explant cultures in plasma clot, the schizonts have been grown successfully for months in tissue culture monolayers3–5 and suspension cultures6. Successive cultivation of schizonts in tissue culture has been shown to bring about attenuation of the parasite7–10, and suspensions of attenuated schizonts have been used as vaccine to immunise cattle against T. annulata in Israel8 and Iran11. Reports8,11 indicate that the protective effect of the vaccine has been tested against challenge with schizonts but not against the natural tick-induced infection. As there may be some difference in the antigens of schizont and sporozoite (the injected stage of the parasite in the tick), we have evaluated the immunity produced by the vaccine against the infection caused by tick infestation.