Ionophores are substances which facilitate the movement of ions into and/or through organic phases. The two major classes of compounds are the neutral ionophores, which form charged clathrate complexes with ions, and the carboxylic polyether ionophores which are neutral in the complexed state. Many of the ionophores exclusively bind organic monovalent cations. Selectivity for divalent metal cations or for anions is less common. One carboxylic polyether, X-537A, forms complexes with small organic cations as well as a variety of metals. Ionophores selective for monovalent cations have been used primarily for studies of mitochondrial metabolism. Compounds like X-537A and A23187, which show high affinity for biologically active divalent cations, are being widely used as tools to study the physiologic roles of these cations. These studies have confirmed the central role of calcium in many biological processes. The major problem encountered with X537A is a lack of selectivity. A23187 has produced variable results in studies of bone as well as other tissues. Despite these difficulties, the ionophores offer promise of being useful tools in studies of bone metabolism.