Monensin A is a polyether antibiotic first isolated from Streptomyces cinnamonensis in 1967. Monensin is a broad- spectrum anticoccicidial antibiotic also exhibiting antifungal and antiviral activity. Monensin A forms complexes with monovalent cations such as Li+, Na+, K+, Rb+, Ag+ and Tl+ and is thus able to transport these cations across lipid membranes of cells, playing an important role as an Na+/H+ antiporter. It blocks intracellular protein transport and is used in the treatment of animals to prevent coccidiosis, promote growth and prevent bloat. Derivatives of monensin, monensin methyl ester and particularly monensin decyl ester, are used in ion selective electrodes.