New polyether antibiotic A82810, its acyl and alkyl ester, acyl ester and urethane derivatives, and salts thereof, are useful antibacterial and anticoccidial agents and increase feed-utilization efficiency in animals. Methods of making A82810 by culture of Actinomodura fibrosa sp. nov. NRRL 18348 and synergistic compositions of the A82810 compounds with nicarbazin, 4,4'-dinitrocarbanilide, certain napthalenamine and benzenamine compounds and metichlorpindol are also provided.