Fully susceptible cross-bred calves, six to nine months of age, were immunised by tick-induced Theileria annulata infection treated with chlortetracycline at 16 mg/kg body weight for four, eight or 16 days. The infections were induced with 10 ticks (Hyalomma anatolicum anatolicum) or 30 ticks (H dromedarii). The recovered calves were tested for immunity to homologous severe challenge, 50 or 73 days after the first infection. The reaction of the calves to infections was evaluated by noting the prepatent period, symptoms, degree of anaemia, rate of parasitisation of lymphocytes and erythrocytes. It was observed that untreated calves developed acute theileriasis characterised by typical symptoms and lesions and 56 to 66 per cent mortality. The medicated calves, however, developed a mild form of the disease. Calves which recovered from treated or untreated infections were solidly resistant to subsequent severe homologous infection. Judged from the severity of anaemia in the infected calves, eight day and 16 day medication provided slightly better protection than four day medication. It was concluded that eight day medication afforded adequate protection against the severe immunising infection, and allowed the development of solid resistance to severe homologous challenge.