An evaluation was made of the protection induced by an attenuated Anaplasma marginale vaccine in young purebred cattle against the challenge exposure of naturally transmitted anaplasmosis in enzootic areas of Mexico. The cattle, which were raised in isolation units free of arthropods, consisted of 10 Brown Swiss calves (1 to 13 months of age) and 8 Holstein calves (5 to 7 months of age). They were paired by breed, age, and body weight, and allotted to 2 equal groups. Calves in 1 group were vaccinated, and at 6 weeks after vaccinations were done, calves in both groups were placed in the field where they were raised for approximately 1 year. Two Holstein and 3 Brown Swiss calves of the nonvaccinated group (group 2) developed clinical anaplasmosis, and the remaining calves of this group had hematologic evidence of the disease during the 2 to 4 months after introduction to the field. The vaccinated group, which remained free of anaplasmosis, showed consistently greater weight gain than did the controls. Among the Holstein calves, the maximum weight difference in favor of the vaccinated group was 50 kg/head at 5.5 months after field exposure, and among the Brown Swiss calves, the differences in weight gain in favor of vaccinated calves at the end of the 12-month period was between 11 and 30 percent. It is concluded that the vaccine provided a means for safe adaptation of high-quality young cattle to the tropics by protecting them against what appears to be the major obstacle to this practice, anaplasmosis.