A long-acting formulation of oxytetracycline (L-200) was injected at a dose level of 20 mg/kg intramuscularly into 12 yearling cattle that had been experimentally inoculated with Anaplasma marginale 64 days previously. Four cattle were treated twice, 4 were treated 3 times, and 4 were treated 4 times. All injections were given once ever 7 days. Previous work (unpublished) with this formulation has revealed prolonged blood values following a single injection. The blood (80 ml) of each calf was inoculated into a susceptible test calf 83 days after the last treatment dose. Anaplasmosis did not occur in any of the test calves. Two infected cattle were treated with 12 daily injections of oxytetracycline (L-50) at a dose level of 11 mg/kg to establish the sensitivity of the parasite to oxytetracycline. Subsequent calf inoculation tests showed that A marginale was eliminated. Two unmedicated, infected cattle remained infected throughout the study. Complement-fixation tests with A marginale antigen at 6 months after treatment showed average sera titers of less than 5 in the treated cattle and 45 in the unmedicated cattle.