Nigericin is a polyether antibiotic produced by Streptomyces, notably S. hygroscopicus, isolated in the 1950s. Its complex structure was finally elucidated in 1968. Nigericin is an ionophore, possessing very high affinity for monovalent cations such as Na+ and K+. Nigericin disrupts membrane potential and Golgi apparatus in mitochondria. Although nigericin can be isolated as the free acid (under acidic conditions), like most ionophores it is extracted into organic solvents and is most conveniently isolated as a salt. In vitro, nigericin has broad biological activity against Gram positive bacteria, fungi, tumor cell lines and some viruses, including HIV. Nigericin is the most common member of the polyether class which are common false positives in in vitro screening bioassays using crude microbial extracts. They are thus important standards for dereplication.