The efficacy of three immunization methods for bovine anaplasmosis was tested on 88 yearling Normandy cattle which were challenge-exposed under field conditions in an enzootic zone in Colombia, South American. A total of 30 cattle were immunized (premunized) with a presumed mild Colombian isolate of Anaplasma marginale; 29 calves were vaccinated with an attenuated A marginale of ovine origin; and 29 were exposed to a virulent A marginale stabilate of Colombian origin. Twenty-nine nonvaccinated cattle served as controls. Parasitemia, packed cell volume, body weight, and complement-fixation antibody response were monitored to determine host response to challenge exposure. The calves experienced a Babesia challenge exposure following arrival in the enzootic zone, after which a transient increase in Anaplasma parasitemia, accompanied by a decrease in packed cell volume, occurred in most of the vaccinated calves. However, neither clinical anaplasmosis nor mortality due to the disease occurred in the immunized cattle. Twenty-seven of the nonvaccinated calves had signs of clinical anaplasmosis and five (17%) died of the disease.