Okadaic acid is a naturally occurring polyether toxin that was originally derived from marine dinoflagellates, Prorocentrium spp.. It is a reversible, potent and selective inhibitor of two serine threonine protein phosphatases: PP2A-C (PP2A) which is inhibited completely at 1 nM and PP1 which is inhibited at higher concentrations (IC50= 10-15 nM). PP2B is much less sensitive to okadaic acid than PP1, while PP2C is not inhibited. This selectivity is the basis for an improved identification and quantification procedure for these enzymes. The hydrophobic backbone of okadaic acid enables it to enter cells where it stimulates intracellular protein phosphorylation. It mimics the effects of insulin, enhances transmitter release at neuromuscular junctions, causes vasodilation and is a very potent tumor promoter. Okadaic acid is an extremely useful tool for studying cellular processes that are regulated by phosphorylation. Okadaic Acid is an activator of PKC.