A new factor that not only inhibits brain microtubule assembly but also causes depolymerization of pre-assembled microtubules was purified from unfertilized sea urchin egg cortices. The purified fraction contained neither proteins nor nucleic acids. The inhibitory activity of the fraction was heat-stable and insensitive to various proteases, but decreased on treatment with glycosidases or with periodic acid. The purified factor was identified as a polysaccharide by analysis of the sugar composition. The sedimentation constant of the factor was estimated to be 9S by sucrose density gradient centrifugation. Polyacrylamide gel electrophoresis of the factor revealed a single broad band stained with periodic acid-Schiff reagent. The factor completely inhibited microtubule assembly at a factor: microtubule protein ratio of 0.03 (w/w). The polysaccharide was found to bind to microtubule-associated proteins, thereby inhibiting microtubule assembly.