Levuglandin E2 (LGE2) is a gamma-keto aldehyde produced by rearrangement of the prostaglandin endoperoxide PGH2 under the aqueous conditions of its biosynthesis. We show that exogenous LGE2 enters cells and efficiently inhibits the first synchronous cell division of fertilized sea urchin eggs. We attribute this inhibition to covalent modification of tubulin and thereby to inhibition of microtubule assembly.