Abstract: Disclosed herein are new salts and polymorphs of cethromycin for the treatment of diseases due to infection by bacteria and certain protozoans, including, for example, malaria, Babesosis, Toxoplasmosis, diarrheal disease, respiratory disease, sexually transmitted bacterial infections, and some bioterror bacteria, including, for example, plague, tularemia and post-inhalation anthrax. Also disclosed herein are new salts and polymorphs of cethromycin for the treatment of inflammatory diseases, including, for example, pelvic inflammatory disease, and peptic ulcer disease.