Discovered in the late 1940s, the pyrrolinonodithioles represent a family of potent disulfide-containing natural products. Although they are understood in a synthetic and biosynthetic context, the biological role of these materials remains unresolved. To date, their activity has been suggested to arise through regulating RNA metabolism, and more recently they have been suggested to function as backup thiols for detoxification. Using materials identified through a natural products program, we now identify the biological function of one member of this family, pyrroloformamide, as an antimitotic agent acting, in part, by disrupting cytokinesis.