Metablastin (also called p19, stathmin, prosolin, p18, Lap18, and oncoprotein 18) is a highly conserved, cytosolic 149-amino acid polypeptide that is expressed in immature vertebrate cells and undergoes extracellular factor- and cell cycle-regulated serine phosphorylation. The protein was shown recently to destabilize microtubules in vitro (Belmont, L., and Mitchison, T. J. (1996) Cell 84, 623-631). Here we demonstrate that microinjection of recombinant metablastin induces a loss of microtubules in COS-7 cells. This effect is enhanced by serine-to-alanine mutations at several phosphorylation sites and virtually abolished by aspartate substitution at a single site, Ser-63. We also show that stoichiometric amounts of metablastin prevent assembly and promote disassembly of microtubules in vitro. Interestingly, the phosphorylation site mutations of metablastin that have dramatic differential effects in intact cells do not alter the ability of metablastin to block tubulin assembly in vitro. The data suggest that phosphorylation of metablastin controls its microtubule-destabilizing activity in vivo but that this regulation may require additional cellular factors. This control mechanism is poised to play a critical role in the dynamic reorganization of the cellular microtubule network that occurs during morphogenesis and mitosis.