Latrunculin A, a toxin purified from the red sea sponge Latrunculia magnifica, was found previously to induce striking reversible changes in the morphology of mammalian cells in culture and to disrupt the organization of their microfilaments. We now provide evidence that latrunculin A affects the polymerization of pure actin in vitro in a manner consistent with the formation of a 1:1 molar complex between latrunculin A and G-actin. The equilibrium dissociation constant (Kd) for the reaction in vitro is about 0.2 microM whereas the effects of the drug on cultured cells are detectable at concentrations in the medium of 0.1-1 microM.