A clinical trial testing the prophylactic effect of a 5 mg kg-1 dose of buparvaquone on either Theileria annulata or Theileria parva experimental infections of calves demonstrated its efficacy for periods of at least seven days. The drug given 1 h or seven days before 50% lethal T. annulata sporozoite infection protected all eight calves, but prophylaxis was insufficient after 14 days to protect two out of four calves from severe reaction. When immunity was challenged by a lethal second parasite dose a month after the first, all these calves were immune. In the T. parva trial, calves given drug 1 h or seven days before a 25% lethal infection underwent minimal reaction, but some were over-protected and were susceptible to a similar challenge sporozoite dose. Although drug levels remaining 14 days after prophylaxis protected these calves from the mild challenge, some parameters measured were within the range of the 'no drug' control group. These results indicated the effectiveness of a single 5 mg kg-1 dose of buparvaquone for more than seven days but also the potential risk of its use in the infection and treatment method of immunisation. It is suggested that there may be circumstances where simple field prophylactic treatment with buparvaquone may be beneficial.