The rational development of new antineoplastic agents directed against tubulin, a protein critical for cell division, requires greater understanding of the interactions between the polypeptide subunits of tubulin and its two tightly bound guanine nucleotides. We have continued our studies on agents which stabilize this very unstable protein, and have found that several of these stabilizing agents also have the property of inducing tubulin to polymerize into microtubules in a reaction requiring no further addition except GTP. We have employed the most economical of these stabilizing agents, the amino acid glutamate, to develop a rapid, large-scale purification of tubulin. The purified protein is electrophoretically homogeneous and free of nucleoside diphosphate kinase and ATPase activities. The polymerization and GTPase activities of the purified tubulin in the glutamate-dependent system were examined. Finally, preliminary studies of the effect of analogues of GTP on the polymerization reaction were begun.