Ionomycin is an ionophore produced by the bacterium Streptomyces conglobatus. The molecules act as a motile Ca2+ carrier and enhances Ca2+ influx by direct stimulation of store-regulated cation entry across biological membranes. It is highly specific for divalent cations. Ion selectivity is as follows: Ca2+ > Mg2+ >> Sr2+ = Ba2+ Binding of Sr2+ and Ba2+ is insignificant and binding to monovalent cations or rubidium is negligible. La2+ is also bound to some extent. Complexation with a cation is always in a 1:1 stoichiometry and pH-dependent. Essentially no binding of Ca2+ occurs below pH 7.0 and maximum binding takes place at pH 9.5. At the micromolar level, ionomycin can activate Ca2+/Calmodulin dependent kinase and phosphatase to stimulate gene expression. Ionomycin has been shown to induce central demyelination, inhibit adrenal bovine TREK-1 channels, and to regulate cell division of mature human B cells [1]. It is used to study the effects of calcium flux on endoplasmic reticulum (ER) stress, mitochodrial stress and intrinsic apoptosis mechanisms. It is also used to stimulate the intracellular production of the cytokines, interferon, perforin, IL-2, and IL-4 usually in conjunction with PMA.