Anaplasma marginale is one of the most prevalent tick-borne pathogens of cattle worldwide. Cattle that recover from acute anaplasmosis become carriers in which low or microscopically undetectable A. marginale rickettsemia persists. There are currently no antimicrobials approved for elimination of persistent infections. In this report we tested the efficacy of various antimicrobial treatment regimens to clear persistent A. marginale infections. In the first study, administration of oxytetracycline at 30 mg/kg, intramuscularly (IM) once or twice 5 days apart was not effective for elimination of the persistent infections. Furthermore, the study demonstrated that the current recommended OIE treatment protocol of 5 injections of oxytetracycline administered at 22 mg/kg intravenously (IV) was also ineffective. In the second study we used flow cytometric analysis to evaluate the effect of antimicrobials against A. marginale in short-term whole erythrocyte cultures. Enrofloxacin inhibited A. marginale in a concentration dependent manner, while higher concentrations of imidocarb were less effective in reducing the number of viable organisms. Oxytetracycline was found to be the least efficacious antimicrobial in this culture system. Cultures of erythrocytes infected with the Oklahoma isolate exposed to 4.0 mug/ml enrofloxacin and those of the Virginia and Oklahoma isolates exposed to 1.0 mug/ml appeared to be sterilized. Cultures exposed to 16 mug/ml oxytetracycline were not sterilized. As a result of these data we tested the efficacy of enrofloxacin (BaytrilRTM100, Bayer Animal Health) against severe experimental A. marginale infections in splenectomized calves. These data indicate that enrofloxacin administered at 12.5 mg/kg twice, 48 hours apart ameliorates, but does not clear, A. marginale infection in splenectomized calves. Finally we compared the efficacy of enrofloxacin, imidocarb and oxytetracycline against persistent Anaplasma marginale infections. The results suggest that apparent clearance of persistent infections occurred in one calf treated with imidocarb dipropionate at 5 mg/kg administered IM twice, 7 days apart, and one calf treated with oxytetracycline administered at 22 mg/kg g24h for 5 days. No calves treated with 5 mg/kg intravenous enrofloxacin for 5 days were cleared. Further studies are warranted to investigate whether dose regimens can be identified to reliably eliminate persistent A. marginale infections.