A new class of mitotic inhibitors is described. Diphenylhydantoin and 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), its metabolite, increase the mitotic index of cultured human lymphocytes without affecting the second resting phase (G2). Mitotic inhibition by these compounds is time dependent, and not a property of hydantoins in general. HPPH proved to be more potent than diphenylhydantoin. Electron micrographs of HPPH-treated human lymphocytes showed changes in 45% of the cells in metaphase: a decrease in microtubules or the appearance of 100 A microfilaments. These changes resemble those induced by colchicine.