Seven cattle, immunized by an infection and treatment method with 3 strains of Theileria parva (Muguga, Kiambu 5 and Serengeti transformed), and five controls were exposed to natural tick infestation in Tanzania, at a site where 2 precontrols had previously contracted fatal East Coast fever within 3 weeks. All controls became infected with fatal ECF within a month of exposure. One immunized animal survived, two died from a Theileria mutans infection and one from a Babesia bigemina infection. The cause of death of the other three animals could not be ascertained, but East Coast fever could not be incriminated. Immunized cattle took longer to succumb than the controls, and serological results showed that all immunized animals became infected with T. parva during exposure, without showing symptoms of ECF.